Once away from a heat source, the residual heat energy absorbed by thermal protective garments can continue to transfer and can injure human skin. Such a post-fire exposure period should not be neglected when investigating the thermal protective ability of thermal protective garments. In our paper, a heat transfer model of a microsystem consisting of a thermal protective garment, an air gap and multilayer human skin is established, and numerically solved via the Finite Element Method. Temperature distributions of the microsystem during fire exposure and post-fire exposure are extracted from the model, and the thermal behavior of skin underneath the thermal protective garment is elucidated. Two parameters [retardation time (Δt) and temperature maximum (Tmax)] are proposed and employed to assess the thermal response of human skin during the post-fire exposure period. Furthermore, the effect of several factors (heat source intensity, thickness and thermophysical properties of the garment, and air gap thickness) on the two parameters and the thermal behavior of human skin are investigated and compared. This might be a significant reminder of self-protection for firefighters on duty, and informative or thermal protective garment design.