In this paper, we derive a representation for the Drazin inverse of a block matrix M = (A B C D) under the assumptions AA π B = 0, CA π B = 0, AA D BSS π = 0, SS D CWAA D (AW) π = 0 and (AW) π AA D BSS D = 0, where S = D − CA D B is the generalized Schur complement. And the representation can be regarded as an unified form of M D because it covers the case either S is nonsingular or zero. Moreover, some alternative representations for the Drazin inverse are presented. Several situations are analyzed and recent results are generalized.