Electromagnetic environment assessment is an important step in the design process of a wireless power transfer (WPT) system to ensure that it complies with electromagnetic radiation standards. Electromagnetic environment assessment usually requires an effective magnetic field analytical model. However, the traditional magnetic field analytical model does not consider the impact of shielding media on the external electromagnetic environment of the WPT system. Therefore, based on subdomain analysis technology in the Cartesian coordinate system, this paper establishes a three-dimensional magnetic field analytical model that comprehensively considers coil parameters, passive shielding media, and active shielding coils. To solve the problem of re-dividing various regions when migrating, a misalignment parameter was introduced in the modeling analysis. In addition, an efficient active shielding coil modeling method is proposed based on electromagnetic theory, which reduces the analytical calculation time of the model. Finally, a 200 W WPT system electromagnetic environment assessment platform that includes both active and passive shielding is built. Under different misalignment and transmission distance conditions, the maximum error of the model calculation and finite element analysis (FEA) is less than 3%, and the maximum error of the model calculation and measurement values is less than 8%. Research results verify the accuracy of the proposed three-dimensional magnetic field analytical model and the effectiveness of the shielding measures. This paper lays a theoretical foundation for the electromagnetic environment assessment of WPT systems and the pre-design of shielding measures.