Abstract: According to the leakage fault of high-pressure cylinder of a condensing steam turbine, the change of through-flow parameters during the fault process was analysed, the steam flow rates of different stage groups were calculated by Flugel formula, and the steam leakage was estimated. Additionally, the leakage process was simplified to isentropic expansion process, the irregularity of the leakage point was ignored, the leakage point was equivalent to an ideal nozzle, and the flow rate of the ideal nozzle was calculated. Results show that, the error between the estimation result of Flugel formula and the leakage of the dropped plug is within the allowable range of engineering. The research results provide a novel computational approach for fault calculation in turbine through-flow components, diverging from conventional methods which only use Flugel formula for flow rate estimation. The difference of flow rate of different stage groups is innovatively used to calculate the leakage of through-flow abnormal part. This method provides a functional assistance for fault location and fault component determination.