In order to analyze the interaction of AC/DC systems and its influence on protective relaying， a power grid simulation platform with multi-infeed DC is established based on the power grid of Guangdong Province with EMTDC. The principles and methods of the models adoption， the scale of power system， network simplification and data processing are studied to balance the simulation demands between precision and efficiency. Particularly， the reasonable treatment scheme to Tianguang HVDC system's equivalence access of 500 kV mainframe grid is presented， whose actual infeed point locates on 220 kV AC power system. Besides， the platform represents the actual cases by contracting simulation results with the actual power grid in steady and transient state， which reflects dynamic characteristics of main frame structure with multiple HVDC system and illustrates the effectiveness and credibility of the effort mentioned. In this platform， a large amount of different fault cases possibly causing commutation failure or transient power converse are simulated and analyzed， which determines the impact range of the interaction of AC/DC systems. It demonstrates the possible application of the platform and affordable means to study different cases of these problems.