Abstract: Aiming at the problem that the value range of traditional fairness metrics is affected by the number of users, this paper proposes a fairness index based on rate variance, which has a fixed value range and high sensitivity. Based on the proposed fairness index, the power allocation problem constrained by intra-group and inter-group fairness was studied in non-orthogonal multiple access systems. This paper decoupled the problem into an inter-group power allocation problem and an intra-group power allocation problem. A joint intra-group and inter-group power allocation algorithm for maximizing the total throughput under fairness constraints was proposed. The simulation results demonstrate that this algorithm can take into account the intra-group, inter-group and system fairness, and maximize the system throughput under the premise of ensuring fairness.