Model checking based on decision diagram causes memory explosion in integer multiplier verification. An efficient solution to this problem is backward substitution method. As the kernel of backward substitution method, the performance of function substitution algorithm is crucial to the efficiency of verification process using backward substitution method. If the variable to be substituted is ensured to be the top variable of the function to be substituted, the function substitution algorithm can be simplified. By setting variable order and variable substitution order, an improved backward substitution method is presented, where in each substitution, the variable substituted is the top variable of the decision diagram of function to be substituted. As experiment shows, under 1GB memory, the improved method enhances the scale of Add-Step multiplier which can be verified from 84×84 bits to 256×256 bits，and Diagonal multiplier from 84×84 bits to 206×206 bits.