In order to maximize the effective throughput while protecting the primary user，a subchannel allocation algorithm based on the subchannels collision probability restraints is proposed by considering the collision between primary user and cognitive user caused by primary user’s return. The proposed algorithm protects the primary users by constraining the subchannels collision probability, and the maximum transmission delay that each subchannel can tolerate is estimated by the probability of collision restraints. The appropriate subchannel is selected according to the cognitive user's service. Simulation results show that the allocation algorithm can protect the primary users well, and can achieve better throughput performance under better channel conditions, compared to the algorithm without considering the constraints of the subchannel collision probability.