In the case of loss of main power and all off-site AC power sources at a nuclear power plant (NPP), each emergency diesel generator (EDG) should be capable of meeting the power requirements for normal operation of all loads necessary to bring and maintain the plant in a safe shutdown condition and prevent damage to critical equipment due to the loss of off-site power. Therefore, it is necessary to study the transient characteristics during the reloading process of EDG to guide the equipment selection and load distribution design. This paper introduces the configuration of EDG, presents the mathematical models of the diesel-governor control system, excitation system, and provide the equivalent circuit diagram of synchronous generator, and electrical loads. A simulation model is established based on ETAP software to realize the simulation calculation of EDG’s reloading process under different operating conditions. The change curves of generator terminal frequency and voltage values during EDG’s reloading process are provided. The simulation results show that the selected equipment and reloading program have good dynamic characteristics and stability. This paper provides a reference for the study of transient characteristics during EDG reloading and guidance for equipment selection and load distribution of EDG.