Abstract: Based on the idea of contention reservation access and polling transmission, a modified User-dependent Perfect-scheduling Multiple Access (UPMA) protocol is proposed, the bandwidth can be allocated dynamically to the uplink and downlink according to the service requirement, the active node can access the channel rapidly by effective collision resolution scheme, and no transmission overhead will be wasted during transmission by the valid polling scheme. Meanwhile, the theoretic analytical model is proposed and its correctness is validated by simulation. The theoretic value and the simulation results show that the proposed protocol has a high throughput, low information delay and small information dropping probability. Finally, with the theoretic analysis, to a given length of polling period, the optimal length of contention access period is given.