NOVELTY - Device comprises alkalinity online monitoring system is useful for giving an early warning in time when the buffer capacity drops in the fermentation process and automatic buffer solution supplementing system useful in conducting rapid adjustment to effectively prevent acidification in the mixed fermentation process.    USE - The device useful in preventing acidification during mixed anaerobic fermentation by automatically regulating and controlling alkalinity (claimed).    ADVANTAGE - The device; is easy and convenient to operate, economical, safe and capable of saving manpower greatly; increases the economic benefits of mixed fermentation engineering; and improves the production efficiency of the mixed fermentation engineering.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is also included for preventing acidification during mixed anaerobic fermentation by automatically regulating and controlling alkalinity comprising (i) in the anaerobic fermentation tank adding straw and human and animal feaces or other organic waste of two kinds or three kinds of mixed material, adjusting the pH before starting the reaction between 7-7.5, keeping the solid concentration of 8%, heating is maintained at 37 plus minus 1 degrees C, (ii) starting the reaction, stirring the mixture uniformly for mixing of materials, setting the anaerobic fermentation tank in the lower part of alkalinity online monitoring data of the three the electrode A, B, C and real-time detection of materials, (iii) the pH value online monitoring electrode A, B and C data testing signal is provided with data collecting card in the controller, the controller statistically analysis and then the liquid crystal is displayed on the screen of the controller, noting the display value and the mean value of the third point, drawing out instant fluctuation curve, (iv) when the alkali degree value is in the range of 1000-2000 mg/l carrying out step (v) or step (vi), buzzer controller and buffer solution is in static state, carrying out step (iii), step (vi) the controller provides a buzzer signal, a buzzer sound is the early warning, when when alkali degree mean value is less than 1000 mg/l, detecting pH value display and carrying out step (vii), and (vii) the controller to buffer solution controller give out signal, controller activates buffer solution, buffer solution to the electromagnetic valve controller in fluid infusion tank give out light signal and accordingly detects the value by the flow rate and time accumulated value through the liquid inlet pipe to anaerobic fermentation tank, adding buffer solution, recovering the normal range value in (1000-2000 mg/l), one time is finished and buffer solution play role in 10 minute cycle and pH value controller gives out working signal and returning to step (iii).    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic representation of the device useful in preventing acidification during mixed anaerobic fermentation by automatically regulating and controlling alkalinity.