Abstract: In order to improve the economy of a SGT5-4000F gas-steam combined cycle unit, the warm-up load optimization was adopted. Combined with the intermediate pressure bypass control mode of the unit, abnormal cases of steam turbine startup after optimization were analyzed. Results show that, the warm-up load optimization method can indeed shorten the startup time of the unit, but at the same time, this method will cause abnormal cases in which the steam turbine cannot startup in summer. The steam turbine can startup normally by increasing the warm-up load of gas turbine or reducing the upper limit of the intermediate pressure bypass under automatic setpoint control mode（ASA control mode） appropriately.