Abstract: The control mode of the steam turbine main steam bypass system of M701F gassteam combined cycle unit manufactured by the Japan Mitsubishi was introduced. Combining with the matched action process of the bypass valve and the main steam control valve during the unit starting up process, the reasons of the fast falling of the water level in middlepressure steam drum were analyzed. The preventive measures and operation proposal were put forward. After adjustment of open speed of the high and middle pressure steam adjusting valve, the fluctuation of the water level in steam drum was decreased effectively, and the unit safe startup and shutdown were ensured. 
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Key words: gas and steam combined cycle unit, bypass control, water level in steam drum