Vapor seal is an indispensable component in steam turbine for economic and safe operation. The flow characteristics in vapor seal have a significant effect on the performance of vapor seal. Considering the flow field characteristics and engineering applications, the following work has been done: First, the models of the labyrinth glands with stepped teeth have been established through Computational Fluid Dynamics software Fluent. The full-flow simulation result is acquired. Second, based on the numerical results, the characteristics of the flow field in stepped labyrinth seal have been studied. At last, the best structure for stepped labyrinth seal is achieved.