Accurate rotor position is essential for PMSM to execute vector control. The resolver and encoder have been most commonly employed, however, the bulk mass usually limit their further application in the fields of aerospace and robotic joints. This paper presents a PMSM rotor position sensor with minimum components, including linear Hall sensors and an optimized permanent magnetic ring which can produce a sinusoidal air-gap magnetic field. The working principle is discussed, and the errors and its compensation are analyzed. Experimental results proved that the presented rotor position sensor has high accuracy and strong anti-temperature drift performance, and the position error can be controlled in desired range by means of Kalman filter (KF) and extended Kalman filter (EKF).