Based on the theory of electromagnetic waves, the surface impedance method is a finite element method used surface impedance as a boundary condition, whose advantages are of low calculation and high computational efficiency. Firstly, Magnet is used to simulate the model by finite element method and surface impedance. Then, based on the P21B model in the TEAM Problem 21 reference family, a stray loss test system is built. The stray loss is measured under different current excitation conditions. The temperature coefficient is introduced to improve the measurement, and the improved coil loss method is adopted to obtain more accurate stray loss measurement results. The calculation results of the finite element method and the surface impedance are verified with experimental measurements. The surface impedance is much smaller than the finite element method in terms of calculation scale and time, which greatly saves computational resources.