Microbial ecological agent is a kind of live bacteria agent widely used in dairy cows industry for its important effect with its trait that free from toxin, contamination and residue. Microbial ecological agent could promote the proliferation of rumen microorganism and increase the production of ammonia，organic acid and mycoprotein in rumen，then improve the environment pH，stimulate the growth and activity of the rumen microorganism，increase its zymohydrolysis on feed，then enhance digestion and increase feed conversion rate. This paper reviewed the effect of microbial ecological agent in dairy cows industry and matters need attention in the application.