The invention discloses a steam turbine bypass control system having a pressure regulating function and a control method. A steam turbine is one of the main equipment for power generation in a gas-steam combined cycle power generation project, and transforms steam heat energy into rotating mechanical energy. The steam turbine bypass control system having a pressure regulating function comprises a boiler, wherein the boiler is connected with a high-pressure cylinder (2) and a medium-pressure cylinder (3); the medium-pressure cylinder is connected with a low-pressure cylinder (4); the low-pressure cylinder is connected with a steam condenser (5); the lower part of the steam condenser is connected with a condensate pump (12); the condensate pump is connected with a condensate desalting device (11); the condensate desalting device is connected with a booster pump (10); the booster pump is connected with a low-pressure heater (9); the low-pressure heater is connected with a deaerator (6); the deaerator is connected with a water feed pump (8); the water feed pump is connected with a high-pressure heater (7); and the high-pressure heater is connected with the boiler. The steam turbine bypass control system and the control method disclosed by the invention are applied to a power generating turbine.