For the problem of large motor protection in the nuclear station, the stability check method of high impedance differential protection for large motors is presented, and we analysze the effect of CT excitation inflection point, CT number of turns ratio and CT loop cable resistance for the high impedance differential protection of large motors. The test method of stability check for the high impedance differential protection is also introduced, and the setting calculation of evaluation is completed, according to the actual motor dynamic simulation test. At last, the conclusion was obtained by analyzing the test data.