The rotor’s position of brushless synchronous motor can only be got by windings’ currents, terminal voltages in the field where the sensors can not be installed. How to steadily drive the motor in the weak feedback becomes the key to the control method. Quantitative analysis of stability for control method is made for two kinds of motor which has trapezia wave and sine wave anti electromotive force， in which the concept of equivalent current bundle is brought forward. The computer simulation proves the results of analysis. Academic direction is made for the stable control of sensorless driving.