Abstract: The applicability of thin slot formalisms(TSFs) in finite-difference time-domain(FDTD) simulation is investigated by comparing the average relative error of different TSFs to fine-gridded FDTD in terms of slot width occupancy and frequency, and by comparing the electric field distribution inside and outside the thin slot. On this basis, the TSFs are employed to predict the shielding effectiveness(SE) of metallic enclosure with thin slot and of an infinite metallic plate with an overlapped rectangular thin slot. Results demonstrate great improvement of computational efficiency caused by applying TSFs in FDTD simulation of structures containing complex thin slot, which are widely used in engineering.