The traditional rotor position detection scheme is to mount Hall sensors on the motor stator or at the bottom of the motor, and calculate the rotor position by detecting the air gap magnetic field or permanent magnet leakage. However, armature reaction has great influence on the accuracy of rotor position detection. Thus, it is proposed to mount the Hall sensor on the printed circuit boards (PCB) outside the external rotor permanent magnet synchronous motor (PMSM). The permanent magnet of the motor is slightly higher than the rotor yoke, and the rotor position is detected by detecting the magnetic field of the permanent magnet, which is almost unaffected by the armature reaction. Theoretical and experimental analysis found that when the Hall sensors are mounted on the PCB at an interval of 90°, the fundamental wave phase of the two Hall signals is not orthogonal, resulting in an error in the rotor position. To address this problem, it is analyzed that the fundamental wave phase is orthogonal when two Hall sensors are mounted on the PCB at an interval of 90°?Pr/(Pr+1). Then the correctness of the theoretical analysis is verified based on ANSYS Maxwell finite element simulation as well as the physical experiment of the motor, and the feasibility of the position detection scheme is also verified.