[Objective] To overcome the technical limitations of traditional position sensorless control for interior permanent magnet synchronous motor (IPMSM) across the full-speed range, this study aims to develop a position sensorless control strategy capable of achieving stable control across the full-speed range. Specifically, it seeks to address technical challenges such as insufficient observation accuracy of speed and angle at low speeds, significant system chattering at medium and high speeds, and poor stability during speed transitions, thereby achieving smooth position sensorless control switching from low to high speeds. [Methods] This study proposed a full-speed-range position sensorless control strategy based on a multimodal observer. At low speeds, an improved high-frequency square-wave injection method was adopted. By considering cross-coupling effects, a precise mathematical model of the motor was established to effectively extract rotor position information. At medium and high speeds, a novel sliding mode observer was designed, which significantly reduced system chattering by optimizing the switching function and introducing a boundary layer method. To ensure a smooth switching from low to high speeds, a fuzzy logic-based weighted switching algorithm was innovatively proposed, which ensured continuity and stability during switching by dynamically adjusting weight coefficients in real time. [Results] In the low-speed range (0~5% rated speed), the improved high-frequency square-wave injection method limited the position observation error within ±0.05 rad, demonstrating that incorporating cross-coupling effects improved the accuracy of rotor speed and angle estimation. In the medium- and high-speed range (5%~100% rated speed), the novel sliding mode observer effectively achieved position sensorless control and reduced system chattering amplitude by about 60%. After applying the weighted switching algorithm, torque fluctuations during speed switching decreased by 45%, and speed overshoot was maintained within 2%. Compared to traditional methods, the proposed strategy improved control accuracy by over 30% and enhanced dynamic response speed by 25% across the full-speed range. [Conclusion] The proposed full-speed-range position sensorless control strategy effectively addresses the control challenges of IPMSM across the full-speed range. The improved high-frequency square-wave injection method significantly enhances observation accuracy at low speeds, while the novel sliding mode observer efficiently suppresses system chattering at medium and high speeds, and the fuzzy logic-based weighted switching algorithm achieves smooth transitions between speed ranges. Simulation and experimental results fully validate the feasibility and superiority of the proposed strategy, offering a new solution for practical sensorless applications in IPMSM.