At present, several sets of 660 MW units in different regions in China have occurred refusing trip failure of generator process-control reverse power protection in succession. Through the analysis of the action logic and calculation principle of the protection device, under the actual situation of the on-site operation of the system, the device which refused to trip is checked, finding out that the actual operating curve of the protection device in the power plane is greatly deviated from the ideal action curve. According to the principle of reverse power protection algorithm, the deviation is derived from the sampling error of the power factor angle. Therefore, the optimal calculation method of the power factor angle is given from the device manufacturer. And present, an optimal operating strategy for manual trip is proposed for the users who adopt process-control inverse power protection when the reactive power level is very low.