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This paper explored the transition of flow in an elevator shaft and analyzed how the gas flow is affected by the moving elevator car during a building fire. A 3D model was built through ANSYS Fluent, the elevator motion was resolved through dynamic mesh theory. Flow fields in the elevator shaft were compared to describe the flow transition. Pressure distributions were applied to explain how the transition was accomplished and how the gas flow was influenced by the moving elevator car. Chemical reaction in the room released large amounts of CO2 and CO. The change in CO2 and CO concentration in the shaft was applied to measure the influence of elevator motion on the flow. At the start of the simulation, the gas moved slowly and smoothly upwards in the area between the elevator car and the top. As the elevator car moved, this area shrank steadily and significantly. In the end, this area disappeared and the transition of flow status in the entire shaft had been accomplished. The elevator motion decreased the pressure inside the shaft as well as the lobby. While the elevator car moved upwards with 1.75 m/s, the pressure in the lobby was decreased by 142.9％ while the CO2 and CO concentration was increased compared to the case with still elevator cars, which indicated that more fire smoke flew into the lobby when the elevator car moved in the shaft.