A numerical method is presented for calculating shielding effectiveness of the wire-mesh reinforcement. The model is based on the field approximation for thin wire in the finite-difference time-domain (FDTD) method. Then, the method is used to compute the shielding effectiveness of the cage built with wire-mesh reinforcement. The FDTD results are in very good agreement with the results from the method of moment (MOM). Finally, the effects to affect shielding effectiveness of wire-mesh reinforcement are analyzed. These effects include the size of the mesh of the cage, the radii of the wire and the frequency of the incident wave.