While the steam turbine is impulse started, generally the vacuum is lower, there is a part of air in the cylinder and the pipe that has not been exhausted, it will be impulsed to the condenser along with the steam flow while the steam turbine is impluse started. During the moment of impulsing, the steam does not change the heat with copper pipe of the condenser and does not condense, so the vacuum of condenser will always decrease during impulsing. When the steam after the steam turbine is impulse started enters the condenser and begins to condensate and at the same time the air extractor still constantly exhausts the air, then the vacuum can be quickly restored to its original value.