Abstract: Bacillus as a kind of feed additive without residue and drug resistance has been applied more and more extensively.The action mechanism of bacillus was explored from maintaining microcosmic ecological balance in intestinal tract,antagonistic action,digestive enzyme generation,nutrition,enhancing physiological functions and enhancing immunity of animals.Furthermore,the effects of species,distinct individuals,feed nutrition environment and antibiotics on bacillus were investigated in order to provide a basis for its application.