In order to study the magnetic compensation of marine diesel engines，finite element simulation is used for degaussing design. In geomagnetic environment，a finite element simulation model of a diesel engine is established using Ansys Maxwell software，the spatial distribution of the induced magnetic field is calculated，and the magnetic field is measured. The simulation results are compared with the measured results，and the error is no more than 10%，which is within permissible range of engineering application. Compensation coils are used to compensate the magnetic field of the diesel engine. By optimizing the setting of the compensation current，the maximum value of magnetic field of the diesel engine is reduced from 336nT to 50nT，reduced by about 85%.