Aiming at the problem that the result of traditional small range differential ratio braking coefficient test method may not be correct, starting from the test method whose phase is fixed and amplitude is adjusted, its simplified equivalent model is obtained and a new method of small range differential ratio braking coefficient test is proposed according to the test conditions to be fulfilled. The method can ensure that the measured small range differential ratio coefficient is the last fulfilled one among all conditions which bus differential protection action needs to be fulfilled, so that the gotten small range differential ratio coefficient definitely is the actual small range differential ratio braking coefficient threshold of the protection device. The effectiveness and correctness of the proposed method are verified by on-site debugging and test of RCS-915AB whose ratio braking is conventional and BP-2CS whose ratio braking is complex. This work is supported by National Natural Science Foundation of China (No. 51107107).