Abstract: [Objective] Accurate analysis of the air gap magnetic field is the focus of research in the field of precision permanent magnet linear synchronous motor (PMLSM). [Methods] Two-dimensional air gap magnetic field of a secondary trapezoidal Halbach permanent magnet array coreless PMLSM was taken as the research object at the paper. On the basis of the equivalent surface current method, an improved equivalent analytical algorithm with a trapezoidal side length unit was proposed. The equivalent analytical model of the magnetic induction vector of the two-dimensional air gap was established. [Results] And the air gap magnetic field of the trapezoidal Halbach array coreless PMLSM was calculated. The results showed that α and αw have a significant influence on the Bpeak and THDB of the central magnetic field air gap. With the synergy of α and αw, identified the flux convergence effect, which makes the maximum range of Bpeak α>90° and αw<0.5. And the equilateral effect, which causes the minimum region of THDB to change linearly. [Conclusion] The calculation results of the established improved equivalent surface current analytical model in this paper agree with those verified by the finite element method. The calculation is convenient, and the accuracy of the result is high. This research provides a new method for analyzing the air gap magnetic field of a permanent magnet with a nonrectangular cross-section, and also prorides a reference for optimizing the PMLSM pole model.