In most situations, staircase is the only egress to evacuate from high-rise buildings. The merging flow on the stair landing has a great influence on the evacuation efficiency. In this paper, we develop an improved cellular automaton model to describe the merging behavior, and the model is validated by a series of real experiments. It is found that the flow rate of simulation results is similar to the drills, which means that the improved model is reasonable and can be used to describe the merging behavior on stairs. Furthermore, some scenarios with different door locations and building floor numbers are simulated by the model. The results show that (ⅰ) the best door location is next to the upward staircase;(ⅱ) the total evacuation time and the building floor number are linearly related to each other;(ⅲ) the pedestrians on upper floors have a negative influence on the evacuation flow rate.