In recent years, extreme heavy precipitation events have happened and caused enormous losses to underground space at home and abroad. At present, reports about the problem of flood intrusion mechanism and hydrodynamic characteristics in underground space are few and unsystematic. This paper deals with a hydraulic model test of inundation in the passageway of underground space. Through numerous experiments under the various conditions, intrusive flow behaviors in the stairs of underground space are studied in details:The maximum velocity of the staircase is 3.9m/s when the flow discharge reaches to 0.28m3/(s·m). The dray force build-up curves have similar trends, which are all propinquity to liner-growth. The slope of force-discharge relation curve standing by double legs is bigger than the one standing by one leg. The maximum drag force of one leg is 3.93×9.8N while the one of double legs is 8.96×9.8N under the most unfavorable condition. The results provide theoretical significance to guide the normal design and evacuation in underground space and provide reference for the flood invasion mechanism research in the future.