[Objective] To study the effects of different sucrose supplemental levels on in vitro gas production,milk yield and milk composition of lactating cows. [Method] 90（2~3） lactating cows with similar parity,weight and age were randomly selected from a large-scale dairy farm in Balin Zuoqi,Chifeng City,Inner Mongolia Autonomous Region,and were randomly divided into 3 groups：control group,trial group I and trial group II,with 30 cows in each group.The control group was fed regular total mix diet.trial group I and trial group II were fed conventional diets supplemented with 2% and 4% sucrose,respectively,for a period of 65 days.The effects of in vitro gas production,milk yield and milk composition were compared after the trial. [Result] The gas production at 48 hours of fermentation,the theoretical maximum gas production and the gas production rate at 1/2 of the maximum gas production in trial group II were higher than those in trial group I and control group（P<0.05）,and those in trial group I were higher than those in control group（P<0.05）.The milk yield in trial group I and trial group II was slightly higher than that in control group,but there was no difference（P>0.05）.There were no significant differences in the contents of milk components such as fat-to-egg ratio,milk protein percentage,milk fat percentage,urea nitrogen and somatic cell count in control group,trial group I and trial group II（P>0.05）. [Conclusion] Supplementing sucrose to the basal diet of lactating cows can accelerate the rate of gas production in vitro without affecting rumen fermentation mode,and has no effect on milk yield and milk composition,among which 4% sucrose has the best effect.