Prereservation based small buffers is an efficient method for contention resolution in optical burst switching networks. In this paper, the blocking model of prereservation with twoclass state was studied in order to provide differentiated services, and a comprehensive analysis on the blocking performance was exhibited by varying buffer size and traffic ratio. In order to reduce the total loss probability of prereservation with twoclass state, a novel scheme named prereservation based trafficthreshold was proposed by constraining the buffered right of low priority within the specific traffic states. Additionally, through the detailed numercial analysis, the reasonable values of buffer size and trafficthreshold were gained, and the balance of buffer usage between high and low priority was achieved. The simulation results show that comaparing with the conventional prereservation schemes, prereservation based trafficthreshold maintains the total blocking performance in all traffic states and obtains a obvious reduction of burst loss probability in light and moderate traffic states.