Abstract: Non-orthogonal multiple access (NOMA) and ambient backscatter communication (AmBC) are key technologies to improve the spectral efficiency of wireless communication systems. A symbiotic backscatter communication system based on Rayleigh fading channel for downlink NOMA multiplexing is proposed to solve the energy constraint problem. The system consists of one source node S, one backscatter device BD and one destination node D. Considering a practical non-linear energy harvesting model, in the case of satisfying the energy causal constraint of BD, an optimal dynamic reflection coefficient was used to maximize the power of backscattered signal, and an analytical expression of the outage probability of this system was derived. The correctness of the theoretical derivation was verified by simulation. The simulation results show that the loss of BD circuit has an important effect on the outage performance of the system. In addition, the performance of NOMA based symbiotic AmBC system and OMA based symbiotic AmBC system is compared and analyzed by simulation. The simulation results show that NOMA based symbiotic AmBC system has better outage performance.