A 3-D numerical simulation model is developed for inundation analysis in a small underground space. The incompressible fluid analysis method and VOF method are used in the model. The model is verified through comparing some computation results with the experimental data of a full-scale stairs model and of a 1/15-scale basement model of a standard small-scale building comprising two stairs, hallway and five rooms. It is found that the model can well simulate the inundation flow at the bottom of stairs and the inundation process in the basement. It is therefore concluded that the model is applicable to the evacuation analysis in a small underground space.