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The space-based Internet of things(IoT) provides global users with wide coverage, all-weather, large capacity, and other services, which is an important part of the future 6G IoT. However, when the multi-user detection algorithm of sparse code multiple access technology based on the IoT faces many problems, there are also some technical challenges such as the time delay caused by the distance between the user and the base station deployed at the satellite, the performance of high computational complexity and poor convergence performance of the asynchronous multiuser detection algorithm, and so on. In order to solve the problem of multiuser detection in the space-based IoT scenario, the SCMA multiuser detection algorithm is studied in this paper. First of all, a grant-free information transmission framework is designed, and blind detection is used to recognize the user data of active users at the transmitter, and then the asynchronous belief propagation message passing algorithm is used for multiuser detection in the presence of time delay. Finally, the adapted-subspace pursuit algorithm in the field of compressed sensing is used to optimize the multiuser detection algorithm and feasibility verification of engineering application of the multiuser detection algorithm in space-based IoT scenarios by simulation. Simulation results show that the proposed algorithm can effectively improve the symbol error rate of the multiuser detection algorithm for uplink asynchronous SCMA systems, and greatly improve the convergence speed compared with the serial BP-MPA algorithm.