Harmonic State-Space (HSS) modeling, because of its considerable ability in undertaking harmonic analysis, is gaining increasing attention, but the complex modeling process and the large size of model limits its application in power electronic systems. This paper illustrates a convenient method to obtain the open-loop large signal HSS model of a power system by taking the three-phase voltage source converter with SPWM as an example. An HSS reduced order model with the same structure and variables as the original model is proposed based on the three-phase equilibrium relationship. By constructing a converter simulation in Simulink, the effectiveness of the proposed model is verified by comparing with the calculated results of the HSS model. This work is supported by the National Key Research and Development Program of China (No. 2017YEB0903100) and the Science and Technology Project of State Grid Corporation of China (No. 521104170043).