The nuclear power plants are being constructed and operated widely in recent years. Consequently, it is necessary to study nuclear power plants (NPPs) dynamic characteristics and power system dynamics simulations with nuclear plant after NPPs has been introduced into power system. At first, the paper presents functional description of NPP. Then a detailed and new models of generating plant with PWR have been developed in PSASP through user-defined modeling program for medium- term and long-term power system transient stability analyse. In the end, the results for power system dynamics simulation with nuclear plant are presented.