The traditional optimization method of the internal space design of rail transit station directly calculates the passenger flow distribution without zoning the internal function of the station, which results in long delay time of passenger flow. A discrete Firefly algorithm approach to the design optimization of the internal space of rail transit station buildings was proposed. The internal function of the station was partitioned and the passenger flow was further distributed based on the discrete firefly algorithm according to the rail transit station partition, so as to realize the optimal design of the internal space of the rail transit station building. The experimental results show that this method has better capacity of passenger flow in the inner space of rail transit station and is worth popularizing.