It is necessary that formal proof for correctness of relational algebra expressions. It can effectively improve the reliability of database query program. The aim of the present study is to present a proof method for correctness of relational algebra expressions. The basic idea about this proof method is explained as follows. Based on the semantics of relational algebra operators, according to the characteristic of relational algebra expressions, we can formally derivate the semantic of an algebra relational expression. And then we can compare this semantic with the specification. If they are equal logically, the relational algebra expression is correct, otherwise, the relational algebra expression is wrong. This article explains the mean and necessity of proving the correctness of relational algebra expressions. Some rules of formal deduction are presented. Based on it, we present a proof method for correctness of relational algebra expressions. Next, a application is given to illustrate this proof method. At last, conclusions and discussions about this proof method are given. As far as we known, there is no other work which is about proof method for the correctness of a relational algebra expression to the database query specification based on the given semantics.