In this paper, to support services differentiation, the analytical model of delay preemption based priority is developed, and an improved scheme LDPP is proposed to alleviate the over-occupancy of buffer from low priority in heavy load. The composite scheme based on preemption and small buffers is an efficient method for contention resolution. To support services differentiation, it is the first time that the analytical model of delay preemption based priority is built. Further, in order to guarantee the low-loss requirement for high priority bursts, an improved scheme is proposed and investigated by limiting the buffered right of low priority bursts within the specific traffic states.