As a high-quality regulation resource in the new power system, pumped storage units play a significant role in enhancing the reactive power regulation capacity of the power grid and increasing the system's rotational inertia. The grid-connected operation of pumped storage units, through dynamic adjustment of reactive power output, provides crucial support for maintaining the reactive power balance and stabilizing the voltage of the power system. This study conducts multi-dimensional phase regulation capability tests on pumped storage units and quantitatively determines the limit capability indicators for stable phase regulation operation based on the coupling analysis of multiple constraints. The research results provide a scientific basis for optimizing the phase regulation operation strategy of the units, effectively enhancing the reactive power reserve capacity of the power grid, and have significant implications for fully leveraging the phase regulation role of pumped storage units in the power grid and improving the stability of the power system. The test scheme and analysis method introduced in this paper can serve as a reference for the assessment of phase regulation operation capabilities of pumped storage power stations under the jurisdiction of East China.