Compared with the position sensor control, sensorless control avoids a lot of defects caused by the position sensor, at the same time makes the system more stability and stronger anti-interference performance, it has more advantages and meets control field's requirements better in the future, it has gradually become a hotspot in the brushless DC motor control field. The rotor position detection has been a very critical research in the brush less DC motor sensorless control. In this paper, a new type of rotor position detection method is proposed, which is based on the principle and mathematical model of a brush less DC motor after the analysis of advantages and disadvantages of the back EMF principle of zero-detection and the method of the back-EMF superposition, which means a combination of the method of the two rotor position detection, as above. This method neither need a delay of 30 degrees nor require a depth filter, the hardware circuit is also simple and there is no need to compensate for the phase shift. Then, the frequency of the detection signal is three times of fundamental wave frequency in this method and the detection signal is easier to detect when the motor running at low speed, which expand the applicable speed range of the electric motor. This paper analyzes the principle and commutation logic of the rotor position detection method, it come up with a conclusion that the stagnation point of the three BEMF superposition derivative is the commutation points of the stator windings, and it uses the method of the back-EMF zero crossings to determine the current conduction state. The simulation model of the brushless DC motor's control system is established in MATLAB /Simulink, it uses speed and current double close loop PI control. The result shows that the method is correct and feasible, it can get the right rotor position signal and control motor commutation correctly, the method can achieve the control of brush less DC motor without position sensor, and it can provide a reference for further research.