Abstract: Flywheel torque is a key index to measure the rapid response performance of hydropower units, and it is an important basis in the calculation of power station transition process and power system regulation. At present, the load rejection acceleration test method is usually used to measure the flywheel torque of rotating parts of hydropower units domestically to check the design indexes. However, this method has the disadvantages of complex test conditions, safety risks and low measurement accuracy. In view of the operation characteristics of pumped storage units with many operating conditions and frequent switch, this paper puts forward creatively a method to measure the flywheel torque by using time waves of active power and speed during unit’s normal shutdown process from phase modulation condition and by using angular momentum theorem. This method has the technical advantages of easy operation and high measurement accuracy. Besides, it can be conducted simultaneously with the routine debug test or unit’s daily operation, so as to save the test period and labor cost, and avoid the safety risk caused by the load rejection accelerated test method.