Abstract: Taking a PG9351FA gas turbine combined cycle unit as the research object, the unit model was established based on Thermoflex software. The performance of unit under full load and constant load conditions was calculated respectively, and the key parameters of the system were analysed. Results show that, compared with the full load condition and the constant load condition, the increase of ambient temperature has the same effect on the efficiencies of steam turbine, waste heat boiler and combined cycle, but the difference on the efficiency of gas turbine is significant. When the combined cycle unit is operating under constant load condition, a suitable low-grade heat source can be used to improve the inlet air temperature of the gas turbine, so as to improve the efficiency of the gas turbine. At the same time, the back pressure of steam turbine can be reduced to improve the performance of steam turbine by cold end optimization and other measures.