NOVELTY - High organic loading medium-temperature methane fermentation complex inoculum comprises medium-temperature nutritional propionate-oxidizing bacteria and medium-temperature nutritional butyric acid/valeric acid-oxidizing bacteria. The medium-temperature nutritional propionate-oxidizing bacteria comprises Pelotomaculum propionicicum, Pelotomaculum schinkii, Syntrophobacter sulfatireducens, Syntrophobacter fumaroxidans, Syntrophobacter pfennigii and/or Syntrophobacter wolinii.    USE - The complex inoculum is useful in high organic loading medium-temperature methane fermentation system (claimed).    ADVANTAGE - The complex inoculum avoids volatile fatty acid inhibition and increases the operational condition of biogas fermentation system high organic loading condition.    DETAILED DESCRIPTION - High organic loading medium-temperature methane fermentation complex inoculum comprises medium-temperature nutritional propionate-oxidizing bacteria and medium-temperature nutritional butyric acid/valeric acid-oxidizing bacteria. The medium-temperature nutritional propionate-oxidizing bacteria comprises Pelotomaculum propionicicum, Pelotomaculum schinkii, Syntrophobacter sulfatireducens, Syntrophobacter fumaroxidans, Syntrophobacter pfennigii and/or Syntrophobacter wolinii. The medium-temperature nutritional butyric acid/valeric acid-oxidizing bacteria comprises Syntrophomonas wolfei subsp., Syntrophomonas curvata, Syntrophomonas bryantii, Syntrophomonas erecta subsp., Syntrophomonas palmitatica and/or Syntrophus aciditrophicus. The medium-temperature nutritional propionate-oxidizing bacteria and medium-temperature nutritional butyric acid/valeric acid-oxidizing bacteria has: a cell number ratio of 6-9:4-1; and a total concentration of greater than or equal to 1x 109 cells/ml. An INDEPENDENT CLAIM is also included for preparing high organic loading medium-temperature methane fermentation complex inoculum, comprising: (i) respectively culturing medium-temperature nutritional propionate-oxidizing bacteria and medium-temperature nutritional butyric acid/valeric acid-oxidizing bacteria under anaerobic condition to obtain a complex mixed culture broth; (ii) concentrating the mixed culture to greater than or equal to 1x 109 cells/ml with a cell number ratio of 6-9:4-1; and (iii) packing and preserving at 2-8 degrees C.