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Numerical simulation on turbulent flame propagation in premixed gas deflagration process in a tube will be reported in this paper, aiming at identifying the key factors affecting flame shape and flame velocity. Large eddy simulation with premixed gas combustion model is used to obtain results validated by full-scale experimental data. The effect of flow velocity and turbulence on flame propagation is discussed. The flow velocity of premixed gas is observed to be one of the main factors determining flame shape and affecting flame propagation process. The velocity difference of different parts of the flame front, both in magnitude and direction, will lead to tulip-shaped flame. Turbulence would accelerate the propagation of flame periodically. The cause of flame acceleration of low-intensity turbulence originates from two factors, namely, combustion and flow field, which transfer the heat and mass of chemical reaction from diffusion to vortex transport. As high-intensity turbulence will not affect the chemical reaction and the turbulent burning velocity,flame acceleration is controlled only by the characteristics of the flow field.