The invention provides a three-level power allocation method for an SCMA (Sparse Code Multiple Access) system based on maximum capacity, which optimizes the throughput performance of the system, improves the energy utilization rate of the system and belongs to the technical field of communications. The method comprises: step 1, performing power allocation between single user carriers, wherein each user occupies a plurality of subcarriers, and the capacity on the subcarriers occupied by the same user should be kept consistent during allocation; step 2, establishing an optimization model of user intra-group power allocation, iterating the optimization model to obtain optimal allocation power, and performing intra-group user power allocation using the optimal allocation power; step 3, fitting a secondary function according to the optimal allocation power obtained in step 2 and the corresponding capacity thereof; and step 4, establishing an optimization model of inter-group capacity and allocation power according to the parameter value of the obtained secondary function, obtaining the optimal allocation power of each group so that the capacity is maximum, and performing user inter-group power allocation according to the optimal allocation power of each group.