The paper is devoted to the development of the models of magnetic shielding system for the ITER neutral beam injectors. As the reactor operation is quasistationary, the magnetic field reduction system of the injectors combines a passive magnetic shield and a set of active correction coils. Due to the strict restrictions on the field inside the injectors, precision computations are required during the design stage. A special attention has been paid to possible gaps between steel panels of the passive magnetic shields due to manufacturing and assembly inaccuracies. A set of models with different levels of detail has been built for the convergence study. It was shown that the mesh needs to have tens of millions finite elements to provide the required computational accuracy.