An improved user grouping and power allocation strategy was proposed for high complexity problem of optimal user grouping and power allocation schemes for Non-Orthogonal Multiple Access (NOMA) systems. Firstly, the users were grouped, the first user of each subchannel was determined by channel gain value, and the remaining users were allocated by greedy matching method. Then, the power of user was allocated, and the power allocation problem was divided into two parts: inter-subchannel and intra-subchannel. The power was allocated by the linear water-filling algorithm for inter-subchannels, and the power was allocated by the proposed iterative power allocation algorithm for intra-subchannels. Finally, a Lagrangian function was constructed to maximize the throughput of system under the constraints of maximizing transmit power and guaranteeing the minimum data rate for each user. The simulation results show that in the case of multiple users, compared with the LWF-FTPA (Linear WaterFilling-Fractional Transmit Power Allocation) algorithm and EQ-FTPA (EQual-Fractional Transmit Power Allocation) algorithm, the proposed strategy has system throughput increased by 8% and 20% respectively, indicating that the strategy is better than traditional algorithms.