In this paper, the problem of magnetic field compensation for ferromagnetic equipment with permanent magnets is studied, and a mathematical model of the compensation magnetic field is established. After dividing the device into several small cuboids, a mathematical model is established based on the magnetic moment method to simulate the compensation magnetic field. When calculating the coupling coefficient matrix in the model, we use the average of multiple points as the effective value of the coupling coefficient, which improves the reliability and stability of the calculation results. Moreover, for the nonlinear magnetization characteristics of the equipment when the permanent magnet is close to the equipment, the equivalent susceptibility of each unit is solved by optimization method. This method does not need to know the magnetization curve of the ferromagnetic material, which is convenient for calculation and practical application. Finally, through the experimental design and numerical calculation, the magnetic field distribution of the permanent magnet compensation device is obtained. The error between our model calculation result and the actual measurement data is within 11%, which shows that the model can satisfy the industrial requirements. Therefore, the models and calculation method have practical application value.