The contour path (CP) based modeling of thin slots in finite-difference time-domain (FDTD) simulation of complex structures breaks the constraint that the mesh size should be less than the smallest dimension of thin slots to get more accurate results, and therefore greatly reduce the consumption of computing resources. A large-scale parallelization platform JASMIN based modeling method of CP-FDTD is proposed in this paper. The thin slots can be automatically modeled and adaptively allocated in the proposed parallelized CP-FDTD algorithm. The electromagnetic shielding effectiveness (SE) of the metal cavity with different thin slots commonly used in industry is analyzed at the frequency range of 0.05 GHz to 3.00 GHz using the proposed parallel CP-FDTD algorithm. Results show that the SEs predicted using the developed parallel CP-FDTD are in good agreement with those obtained using finely meshed FDTD (fine-FDTD). Moreover, the computational efficiency is significantly improved.