Abstract-In this paper, a fast and efficient method has been proposed to analyze the electromagnetic shielding rooms with electrical large sizes and arbitrary shapes. The ray-tracing method is used to predict the Shielding Effectiveness (SE) of the electromagnetic shielding rooms. The proposed method is based on speeding up the ray tracing algorithm. The performance of the proposed method is verified by a comprehensive example. The effect of additional metal cabinet in the shielding effectiveness of shielding room has been investigated. Also the position of it has been found optimally to produce a "best" performance for the shielding room.