The paper investigates the integration scheme of cell-free massive multiple-input multiple-output （MIMO） and non-orthogonal multiple access （NOMA）. NOMA based on group successive interference cancellation （GSIC） method is applied to the downlink cell-free massive MIMO systems. Furthermore， a novel grouping method according to user’s equivalent path loss is developed and the expression of per user’s achievable rate is derived. The simulation results show that the performance of downlink cell-free massive MIMO-NOMA systems based on GSIC is better than that of the cell-free massive MIMO-NOMA systems based on successive interference cancellation（SIC） in terms of achievable rate.