This paper applies the concept of fault tolerance to staircase layout plan. Fire or smoke may cause some staircases in a building inaccessible. We argue that architects should assess the result from the possible fault of vertical routes. The capability of tolerating staircase faults depends on space usage, arrangement, and pedestrian attributes. In this study, a mathematical model is constructed. For analysing pedestrian's movement in interior space, we employ Monte Carol simulation and Agent-Based Modelling method in a CAAD environment. It helps us to visualise the dynamic process of agent's evacuation process, and to test the problem of possible staircase faults. Finally, a case study brings some important discoveries.