Physical layer security of non-orthogonal multiple access (NOMA) system which uses simultaneous wireless information and power transfer (SWIPT) technique is deeply dicussed in this paper. Generally, eavesdropper in the downlink of NOMA system may use successive interference cancellation technology (SIC) to obtain the secrecy information of receiver. To tackle this problem, we propose a physical layer security scheme to minimize the transmit power of the base station (BS) while the secrecy rates of receivers are guaranteed. Moreover, semidefinite relaxation (SDR) method and successive convex approximation (SCA) technique are combined to solve above non-convex problem. Simulation results show that in comparison with other methods, our method can effectively reduce the transmit power of the BS.