This paper introduces a DSP (digital signal processor) and PC based power system real-time digital simulator dedicated to protective relay testing. The performance requirements, software implementation and hardware architecture are described. The functions of the simulator such as real-time simulation of power system under various fault conditions, harmonic wave generator, fault record replay and simulation of system oscillation are also given. The simulator can be used in the laboratories, power plants, substations and relay manufactures because of its small size and integration of multi-function.