[Objective]To explore the therapeutic effect of microecological preparation on rumen acidosis in dairy cows. [Method]Forty lactating cows were randomly divided into four groups. The control group cows were fed with the basic diet. Group I,Group II and Group III were fed with the basic diet plus 1%,2% and 3% compound microecological agents respectively. The test period was 7 days in the pre test period and 30 days in the formal test period. The growth performance,lactation performance and rumen fermentation parameters of dairy cows were measured. [Result]Compared with the control group,the average daily weight gain and average daily food intake of group I,group II and group III were significantly increased （P<0.05）,the feed to weight ratio was significantly decreased （P<0.05）,and the milk yield and the percentage of milk fat in milk were significantly increased （P<0.05）.The pH and lactic acid content in rumen fluid of dairy cows decreased significantly （P<0.05）, the content of acetic acid, propionic acid and butyric acid in test groups I, II and III increased significantly（P<0.05）,and the ratio of acetic acid to propionic acid decreased significantly（P<0.05）.[Conclusion]Mixed microecological preparation in basic diet of dairy cows can improve growth performance and lactation performance,improve rumen fermentation parameters, and treat rumen acidosis.