Abstract: To solve the chattering problem of traditional sliding mode observer in the process of position estimation, a piecewise square root switching function is proposed to replace the traditional switching function, and its stability is analyzed by using Lyapunov stability criterion. To solve the poor tracking ability caused by the traditional adopted phase-locked loop (PLL) under speed changing conditions, an improved PLL scheme is proposed to improve the recognition speed by adding a differentiation element and to enhance the dynamic performance by adding a speed negative feedback part. Furthermore, a full compensation algorithm is proposed based on the phase-locking and position error identification mechanism, which can solve the problem of position estimation error caused by phase delay links such as filter and zero-order state holder. In addition, the sliding mode observer is established by using the simple d-q coordinate system voltage equation; and when using PLL for position estimation, it is not necessary to construct the position error as in the α-β coordinate system. Simulation and experiments validate the feasibility and effectiveness of the proposed strategy.