In this paper the factors of αp and δp are introduced to describe the effects of optical field longitudinal nonunlformity on threshold conditions. The rate equation model including spatial hole burning is established, and the expression of IM and FM response within small signal approximation are derived. The relationships between threshold condition of ideal DPB laser diodes and the average photon density in laser cavity are obtained numerically. The numerical analysis show that spatial hole burning affects the modulation properties greatly, and FM reponse depends on the bais current. In order to get wider modulation band width proper bais injection current should be chosen.