Abstract: A downlink non-orthogonal multiple access (NOMA) system is studied, in which a base station serves a near user and a far user on the same frequency band simultaneously. Due to physical obstacles or heavy shadowing, there is no direct link from the base station to the far user and the near user acts as a cooperative relay for the far user by adopting the simultaneous wireless information and power transfer (SWIPT) technique. Unlike most existing works, a non-linear model for energy harvesting is adopted to study the system outage performance by considering the non-linear features of the energy harvesting circuits. Specifically, the system outage probability with integral expressions is derived. To characterize the impact of system parameter design on outage probability, the approximated closed-form expression of outage probability can be obtained by further applying the approximation technique, and the optimal design of system parameters is analyzed. Simulation results validate the correctness of the theoretical analysis and the optimal design.