In this paper, we investigated timeslot allocation scheme in medium access control protocol for cognitive satellite networks. The timeslots for satellite primary users were allocated according to the reserve channel with priority fit algorithm and the timeslots for cognitive users were allocated according to the prediction algorithm. Compared to random allocation scheme, the correction ratio of timeslot allocation using the prediction algorithm was improved. The proposed timeslot allocation scheme adopted the combined backoff algorithm to avoid collisions. And then the improved throughput and the reduced delay could be obtained. Simulation results verify the effectiveness of the proposed timeslot allocation scheme.