Abstract: With the development of the new power system, the load centers show a trend of power source "hollowing out, " and the dynamic reactive power support capability is seriously insufficient, posing a threat to the safety of the power grid. Retrofitting the old thermal power units facing standby into synchronous condensers can not only solve the problem of insufficient dynamic reactive power in the power grid but also fully utilize the units' residual value to solve the power plants' operational difficulties. This paper constructs a grid-related performance evaluation system for synchronous condensers retrofitted from large thermal power units, including the steady-state and dynamic performance evaluation methods. It finally obtains the calibrated capacities and dynamic performance grading results after retrofitting the thermal power units. This evaluation system was applied in the demonstration project of the first large thermal power unit retrofitted into a synchronous condenser in China, and the relevant authorities recognized the evaluation results. The evaluation results can serve as the basis for grid departments to arrange production operations and are also the basis for economic compensation for power plants' reactive power auxiliary services, which has important practical significance.