Abstract: Thin slots with complex configurations used in practical engineering make the evaluation of electromagnetic shielding effectiveness(SE) for metallic enclosure increasingly challenging. The contour path (CP) method is adopted to deal with the complex thin slots in order to keep the coarse gridding in FDTD programming. An equivalent model with simple configuration is presented to replace the complex nested slits, and as a result, the programming and calculation efficiency of the CP-FDTD method is improved when dealing with thin slots with complex configurations. The equivalent model and CP-FDTD are used to calculate the SE of metallic enclosure with two typical kinds of thin slot configurations at the wide frequency range respectively. Results are in good agreements with those obtained by using fine-gridded FDTD, and high calculation efficiency is achieved at the same time.