To improve the low-speed performance of sensorless control for interior permanent magnet synchronous motor (IPMSM), a square-wave voltage injection startup control strategy was proposed, utilizing a robust position observer. The square-wave voltage injection method avoided the use of filters, thereby increasing the system bandwidth. Based on the square-wave injection method, a robust rotor position observer was employed to enhance the system’s resistance to load disturbances. The feedback matrix parameters of the observer were designed using pole placement, taking into account practical step and ramp load disturbances as well as desired position observation error indices. Simulations and experiments verified the feasibility and accuracy of the proposed sensorless control strategy.