NOVELTY - A hydrogen sulfide tail gas concentration controlling method involves transferring sulfide and carbon dioxide dissolved thio-containing methanol solution from low temperature methanol washing gas purification absorption unit to middle part of medium-pressure flash evaporation tower to carry out to flash evaporation, followed by passing semi-depleted liquid into upper part of the medium-pressure flash evaporation tower to wash carbon dioxide and hydrogen sulfide and to carry out absorption, and recovering fuel gas such as methane, carbon monoxide and hydrogen dissolved in the solution.    USE - Method for controlling concentration of hydrogen sulfide tail gas in low temperature methanol washing process.    ADVANTAGE - The method enables controlling concentration of hydrogen sulfide tail gas with reduced energy consumption.    DETAILED DESCRIPTION - A hydrogen sulfide tail gas concentration controlling method involves transferring sulfide and carbon dioxide dissolved thio-containing methanol solution from low temperature methanol washing gas purification absorption unit to middle part of medium-pressure flash evaporation tower to carry out to flash evaporation at reduced pressure of 0.5-1.8 Mpa, followed by passing semi-depleted liquid into upper part of the medium-pressure flash evaporation tower to wash carbon dioxide and hydrogen sulfide and to carry out absorption, desorbing and recovering fuel gas such as methane, carbon monoxide and hydrogen dissolved in the solution to fuel gas dispensing device. The methanol solution after the flash evaporation is subjected to pressure reduction and gas stripping to allow a carbon dioxide component to be desorbed from the solution. The methanol solution after gas resorption is entered into middle part of an absorption tower from the bottom of medium-pressure flash evaporation tower to carry out absorption process under reduced pressure of 0.12-0.2 Mpa to release carbon dioxide, hydrogen sulfide and other gases through upper part of the absorption tower, followed by absorbing methanol and semilean solution from upper part of the absorption tower to lower part of a stripping section after hydrogen sulfide gas is desorbed, and to lower part of the low-pressure nitrogen gas absorber to carry out stripping process under low-pressure nitrogen gas pressure of 0.3-0.6 Mpa, passing solution containing hydrogen sulfide rich gas from top of the medium-pressure flash evaporation tower to the lower part of the absorption tower to achieve enrichment of hydrogen sulfide solution, discharging thio methyl alcohol-rich liquid desorbed carbon dioxide from top of the absorption tower, utilizing exhaust gas/hot flash steam heat exchanger to discharge flash vapor, discharging small part of thio-containing methanol solution from bottom of the absorption tower to subsequent flash process, heating thio-containing methanol solution obtained from the absorption tower by pumping hot flash from heat exchanger and heat regeneration tower to 70-85 degrees C. The solution is passed to hot flash evaporation tower to carry out flash evaporation at pressure of 0.2-2 Mpa to obtain steam containing carbon dioxide and sulfide, followed by passing vapor from flash evaporation tower to condenser to change heat to room temperature, returning steam to the absorption tower and to lower portion of the absorption tower, subjecting the thio-containing methanol solution to heat treatment. The thio-containing methanol solution is passed to middle part of the regeneration tower and to flash tank. The resultant solution is passed to middle part of the regeneration tower, carrying out stripping methanol vapor and residual sulfides carbon dioxide dissolved in the solution, discharging top fraction from acid regeneration tower, where the heat required for thermal regeneration is indirectly heated by low pressure steam supplied by reboiler, maintaining the pressure in the thermal regeneration tower at 0.2-0.5 Mpa, and temperature of 85-110 degrees C, discharging lean methanol liquid from bottom of the regeneration tower by using pump and rich methanol heat exchanger, passing sulfur-rich liquid methanol to low temperature methanol washing device, carrying out cooling and refluxing process to obtain the complete regeneration, cooling sulfide fraction, circulating hot regenerated acid gas leaving from the top of the heat regeneration column, circulating the fraction through the heat regeneration condenser using cooing water to 35-45 degrees C, entering to reflux separator, and obtaining acid sulfide tail gas by condensation and cooling through tail gas heat exchanger and a hydrogen sulfide tail gas chiller, reducing the temperature to 25-40 degrees C, and collecting acidic gas fraction entrained methanol condensate separated from the hydrogen sulfide exhaust condensate in a reflux splitter.