The invention provides a method and device for measuring rated guide vane water leakage of a water turbine. The method comprises the following steps: determining reference pressure head on the basis of pressure difference between water pressure in a water inlet pipe of the water turbine and water pressure in a tail water pipe when a flat pressing facility of the water turbine is closed; and determining the rated guide vane water leakage of the water turbine on the basis of the reference pressure head, the area of the horizontal cross section of an inclined shaft section of the water inlet pipe of the water turbine or the area of a ventilating hole of the water turbine. According to the method and device for measuring the rated guide vane water leakage of the water turbine, an algorithm is simple, calculating parameters are less, the water leakage of a gate does not need to be measured, and the testing efficiency of a testing field for the rated guide vane water leakage of the water turbine is improved effectively.