<p>The inverse U-shaped interior permanent magnet synchronous motor (IPMSM) is characterized by low eddy current losses and suitability for high-speed operation, with its unique topology significantly affecting performance. Based on the actual topological structure of this motor, this paper proposes an analytical calculation model of the no-load air-gap magnetic field for the inverse U-shaped IPMSM. On the base of the 2D subdomain method and the principle of constant magnetic flux, four equivalent criteria are proposed to equivalently transform the rotor structure. Since the air-slots in the inverse U-shaped pole structure play a vital role in blocking leakage flux and traditional analytical models ignore them, which results in large errors in leakage flux calculation, this paper specifically considers their influence. By reasonably designing the equivalent structure of the air slots and treating them as an independent subdomain, the calculation accuracy of air-gap magnetic field and back electromotive force is significantly improved. Under boundary conditions, the separation of variables solves the Laplace or Poisson equation of each subdomain, yielding the air-gap flux density analytical solution. The validity of the proposed model is confirmed by comparing analytical results with those obtained from the finite element method (FEM). This work provides theoretical support for the electromagnetic design and performance optimization of such motors, offering a reference for the analytical modeling of high-speed permanent magnet motors.</p>