This paper presents the algebraic structure of the conditional equations set for multi-output Boolean functions,proves that the algebraic immunity of multi-output Boolean functions is equal to that of a certain Boolean function,which is a combination of the component functions for multi-output Boolean functions,and among all combinations,this one has the minimum algebraic immunity.Hence,it can be concluded that the algebraic immunity of multi-output Boolean functions is equal to the minimum of algebraic immunities of all combinations.