In the case of grant free, due to the lack of synchronous time signaling, the problem of asynchronous reception in Sparse Code Multiple Access（SCMA）system cannot be ignored. The existing Belief Propagation Message Passing Algorithm (BP-MPA) can detect asynchronous SCMA, but the complexity is high. Aiming at the problem of high complexity of detection algorithm in uplink asynchronous SCMA system, this paper proposes an Improved Belief Propagation Message Passing Algorithm (IBP-MPA) by selecting some highly reliable user nodes not to participate in the iteration of message passing algorithm according to the judgment of each resource on the probability of user sending codeword. The simulation results show that under the same conditions, IBP-MPA can greatly reduce the complexity of BP-MPA while maintaining the detection performance.