In this experiment,the corn straw fermented by different compound strains was used as the fermentation substrate,and the rumen in vitro fermentation experiment was carried out to determine the relevant indicators,and the optimal compound strains was selected by combining the indicators.The ex-periment was divided into control group(CON),Aspergillus niger + Bacillus licheniformis group(HD),Ba-cillus licheniformis + Phanerochaete chrysosporium group(DB),Aspergillus niger + Phanerochaete chryso-sporium group(HB).Each treatment group had 3 replicates.After 21 days of sealed fermentation at room temperature,the bag was opened and sampled.The fermentation effect was evaluated by sensory evalua-tion and fermentation index.Meanwhile,the in vi-tro rumen fermentation method was used to deter-mine the gas production parameters,fermentation parameters and in vitro digestibility to evaluate the fermentation effect of different compound strains on corn straw.The results showed that the contents of CF,NDF and ADF in HB group and DB group were significantly lower than those in CON group(P<0.05).The activities of acid xyla-nase and β-glucosidase in HB combination were significantly higher than those in CON group(P<0.05)and higher than those in other groups.The activity of external sections of β-1,4-glucanase and filter paper enzyme in DB combination was significantly higher than that in CON group(P<0.05)and higher than that in other.In the rumen in vitro fermentation test,the gas production of DB group was significantly higher than that of CON and HB group(P<0.05).The TVFA of the three experimental groups was significantly higher than that of the CON group(P<0.05),and the DB group was significantly higher than other groups(P<0.05).The in vitro digestibility of acid detergent fiber in DB group was significantly higher than that in CON group(P<0.05).The in vitro digestibility of dry matter,crude protein and neutral detergent fiber in the three experimental groups was significantly higher than that in CON group(P<0.05),and the in vitro digestibility of neutral detergent fi-ber in DB group was significantly higher than that in other groups(P<0.05).In summary,the fermenta-tion of the composite strains selected in this experiment can improve the digestibility of nutrients and the degradation rate of crude fiber in corn straw to varying degrees.After comprehensive analysis,the combi-nation of 5%Bacillus licheniformis + 5%Phanerochaete chrysosporium in corn straw is the best.