For the purpose of testing the performance of GPS receivers, a GPS signal simulator is needed that can emulate the real GPS signals under all kinds of conditions. Single channel and multi-channel GPS signals' characters in time domain and frequency domain are analyed. A mathematic model of the twelve-channel GPS simulator is proposed. In order to reduce the difficulties of the hardware design, this model is designed to provide a 35.42MHz IF signal directly by applying the idea of "software radio" and the theory of interpolation. Simulation results with SystemView software have demonstrated the feasibility of the system scheme. In addition, a practical hardware design of this system is described.