Abstract: Aiming at the secure transmission problem in downlink channel of non-orthogonal multiple access system, this paper proposes a secure scheme aided by cooperative jamming. In this scheme, the base cooperates with external nodes to send the jamming signals to worsen the receiving performance of eavesdroppers. Firstly, we use the Poisson point process to model the location of the cooperative nodes and the eavesdroppers. Then, we derive closed-form solution of the system outage probability under stochastic cooperative jamming scheme. But this scheme with limited performance gain which can affect both the legal users and eavesdroppers. Under this consideration, we further proposed a cooperative jamming scheme based on limited channel gain, this scheme chooses the cooperative nodes which the Rayleigh coefficient is less than a certain threshold, so it will cause more serious impact on the eavesdroppers. Simulation results demonstrate that the limited-cooperative jamming scheme can significantly improve the security performance of the system.