An  in vitro system was conducted to investigate the effects of eight different levels of exogenous complex enzymes in a corn-soybean meal hay diet on rumen fermentation. Exogenous complex enzyme consisted of cellulase, xylanase, acid protease, neutral protease and pectinase. The results showed that gas production, ammonia-N (NH₃-N), volatile fatty acids (VFA) concentration in each treatment were significantly higher than those in control treatment ( P <0.05). It also showed that the rumen fermentation pattern was changed when adding exogenous complex enzyme, and gas production in vitro , NH^₃-N and VFA concentration and their degradability were improved. The best formulation of complex enzyme was determined according to the fermentation parameters, the proportion of cellulase, xylanase, acid protease, neutral protease, pectinase and rice hull meal were 20%, 5%, 5%, 2%, 2% and 66%, respectively.