Abstract: For the low cycle thermal efficiency of Pressurized Water Reactor(PWR) nuclear power units and the growing demand on the nuclear power peaking capacity of the power grid, using Ebsilon software, based on  the thermodynamic system model of the secondary loop of Daya Bay Nuclear Power Station, the nuclear-gas combined cycle power generation thermodynamic system is established. Taking the gas turbine cycle efficiency and the combined cycle efficiency as the index of the heat economy, the performance of the combined cycle system is evaluated. At the same time, the effect of ambient temperature, pressure and gas turbine load on the system performance is analyzed. The results show that, the thermal efficiency of the nuclear-gas combined cycle system increased by 13.15% compared with the original nuclear power unit, the output power of the steam turbine increased by 75.49%, and the working environment of the steam turbine is improved obviously; The fall of ambient temperature will increase the efficiency and the power of the gas turbine, and the drop in ambient pressure will decrease the efficiency of the gas turbine and the combined cycle efficiency. When reducing the load of the gas turbine, the inlet temperature of nuclear reactor remains unchanged by afterburning the natural gas, and the steam turbine can still have high output power. The adjustable range of the load is from 56.57 % to 100%.