In order to meet the power grid adaptability test requirements of medium-voltage direct-connected photovoltaic power stations, energy storage power stations and wind turbines, a 35 kV/3.5 MW grid simulator needs to be developed. Aiming at the problem that the response characteristics of potential transformer (PT) affect the control performance of the system, modeling and characteristic analysis of PT is carried out, and the influence mechanism of PT characteristics on the control performance of the system is demonstrated. According to the circuit topology of the power grid simulator, an improved three-loop control strategy that instantaneous voltage, current feedback on the low voltage side of the transformer and high voltage RMS feedback is presented to improve the dynamic and steady performance of the output voltage of the power grid simulator. The feasibility and effectiveness of the improved control strategy are verified in simulation and prototype.