According to guide on leading power factor operation test of synchronous generator, a capability calculation method of pumped storage unit considering constraints with generator power angle, stator current and machine terminal voltage is proposed. Leading power factor depth limit under generation mode and pumping mode is overall analysed,and the tap changer stalls affecting on the leading power factor depth limit is also studied.Taking # 1 generator of Hongping Pumping Station as example, the Q-PV three-dimensional surface of leading power factor depth limit changing with the system voltage and active power are showed in the picture, leading power factor depth limit of any test condition can correspond to a point in the surface. The test personnel can accurately grasp the leading power factor depth limit under all test conditions through the three-dimensional surface, which can greatly improve the accuracy of leading power factor depth limit assessing.The experimental data are basically consistent with the theoretical calculation results, which validates the correctness and practicability of the proposed method.