The independent decoupling control of two motors in the six-phase series three-phase PMSM system can be realized by controlling the voltage vectors of different planes in the static coordinate system. Based on this, the rotating high frequency voltage is injected into the six-phase plane and the three-phase plane at the same time in the static coordinate system. By demodulation of the negative sequence component of the corresponding plane high frequency current, the preliminary observed rotor position angles of the two motors are obtained by decoupling. To compensate the error caused by stator resistance, speed and filter, the rotor position angle is compensated by combining the error angle information in the positive sequence component of high frequency current with the speed feedforward. The experimental results show that the average absolute value of the steady-state rotor position angle observation error values of six-phase and three-phase motors are 0. 146 and 0. 106 rad, and the maximum error values under dynamic conditions is 0. 2 rad. The sensorless direct torque control system based on the observed rotor position angle has excellent performance in various operating conditions at low speed.