In this paper, we give a computational formula for the Drazin inverse of the sum P + Q, then applying it we give some computational formulas for the Drazin inverse of block matrix M = A B C D (A and D are square) with generalized Schur complement S = D − CA D B is nonsingular under some conditions. These results extend the results about the Drazin inverse of M given by R. Hartwig, X. Li and Y.