During the startup and operation of SIEMENS V94.3A gas-steam combined cycle unit in Xiaoshan power plant, the unit frequently tripped due to drum water level protection. Based on the mechanism of drum water level fluctuation, the root causes for the trip are analyzed in thermal system design, such as water level control system, high & medium pressure bypass system, as well as selection of the feed water pump. As a result, the bypass control is optimized by controlling load speed, and setting the minimum pressure of the intermediate pressure bypass and attemperating function of the high pressure bypass. In addition, the feedwater control is also optimized for successfully control the drum water level. After the optimization, great effect has been achieved and operational reliability in the control system of drum water level is enhanced.