Abstract: Taking a cycle combined with PG9171E gas turbine as the benchmark unit, a gas-steam combined cycle off-design model was established. By changing the ambient temperature and the corresponding boundary conditions, the design/off-design performance and peak load regulation of the combined cycle was analyzed. The results show that, basing the performance guarantee condition as the benchmark condition, with lower temperature as winter condition, the unit exhibits a better off-design performance, with the efficiency increment of 1.69%~3.75%, and its net power output will increase by 11.48%~11.57%. On the contrary, with higher temperature as summer condition, the unit shows lower off-design performance with the efficiency decrement of 2.56%~5.08%, and its net power output will decrease by 10.44%~10.58%. For the peaking load capacity of the unit, with higher temperature the capacity is less and the maximum and minimum generating loads is 47.63 MW and 242.62 MW at -20℃,at 40℃ is 39.14 MW and 172.12 MW respectively; and the peak load in the whole year temperature range is 132~194 MW.