6 Small-tail Han sheep ram,2 to 3 year's old,with body weight of 48.3 kg±5.6 kg,were fitted with permanent rumen fistula and fed with the diet containing 70% of corn stalk.The experiment was designed as 4×4 Latin square,the sheep were orally administered with formalin at 0 mL/kg diet (control,treatment 1),1.0 mL/kg diet (low dose,treatment 2),3.0 mL/kg diet (high dose,treatment 3) and completely fauna-free (treatment 4),to study the effects of various treatments on the rumen microbiocoenosis,activities of digestive enzymes,NH3-N and VFAs in rumen fluid of sheep,and their correlation with voluntary intake.The results showed that,compared with control,the voluntary intakes of dry matter in treatments 1,2 and 3 were increased by 14.9% (P<0.01),-6.8% (P>0.05) and -16.5%(P<0.01),respectively;NH3-N were decreased by 10.5% (P<0.05),13.0% (P<0.05) and 23.0%(P<0.01),respectively;The protozoa number in rumen fluid were decreased by 59.3%,93.2% and 100.0%(P<0.01),respectively;The total number of bacteria were increased by 41.1%,-8.2% and 84.5%(P<0.01),respectively;The fungi copies were increased by 16.5%(P>0.01),23.5% (P<0.05) and 38.1%(P<0.01),respectively;The activities of endocellulase in treatments 1,2 and 3 were increased by 22.6%,-14.6% and -27.5%(P<0.01),respectively;The activities of exocellulase were increased by 15.5%,-23.9% and -25.2%(P<0.01),respectively;The activities of cellobiose were increased by 20.0%,-25.7% and -26.2%(P<0.01),respectively;The activities of xylanase were decreased by 0.4%(P<0.05),0.5%(P<0.05) and 0.1% (P>0.05),respectively;The activities of pectinase were decreased by 12.0%,11.0% and 2.8%(P<0.01),respectively;The activities of amylase were increased by 12.2%,15.3% and 19.9%(P<0.01),respectively;The activities of protease were decreased by 22.1%,48.0% and 22.2%(P<0.01),respectively.The correlation coefficients (R2) of the activities of the endocellulase,exocellulase and cellobiose with voluntary intake of sheep were 0.994,0.897 and 0.901.It was concluded that when the low dose of formalin was orally administrated the voluntary intake of sheep was increased,whereas the high dose of formalin or fauna-free was decreased;The factors determined the voluntary intake of sheep were the activities of endocellulase,exocellulase and cellobiose,rather than the numbers of the protozoa,bacteria or fungi,or the activities of other digestive enzymes in rumen fluid.