The invention provides a method for acquiring efficiency of a low-pressure cylinder of a thermal power generating unit in a wet steam environment. The method includes the steps: obtaining a first exhaust total heat by means of mass and energy balance calculation; acquiring a low-pressure cylinder saturation parameter and the low-pressure cylinder first parameter; obtaining a low-pressure cylinder second parameter through an internal iteration process; obtaining a second exhaust total heat by means of mass and energy balance calculation; repeating the internal iteration process; if the condition of convergence is met and the current back pressure is lower than a critical back pressure, acquiring the efficiency of the low-pressure cylinder under the wet steam condition; if the condition of convergence is not met, returning to the internal iteration process; if the current back pressure is higher than the critical back pressure, replacing the current back pressure by the critical back pressure, and returning to the step of acquiring the low-pressure cylinder saturation parameter under the current back pressure; and alarming when the efficiency of the low-pressure cylinder under the wet steam condition is smaller than a preset efficiency value. Using the method for acquiring efficiency of the low-pressure cylinder of the thermal power generating unit in the wet steam environment can quickly and accurately acquire the efficiency of the low-pressure cylinder of the thermal power generating unit in the wet steam environment and improve system safety.