In this article, premixed methane air flame propagation in a confined vessel at low initial temperature was simulated using a multistep chemical reaction mechanism. The confined vessel was a cylinder with aspect ratio of 3 with asymmetrical position of the ignition source near the side cover. The equivalence ratio and the initial temperature of the premixed unburned combustible gas were 1.0 and 150 K, respectively. The overall evolution of the flame and the flame dynamics were obtained, respectively. Through the entire flow field variation, vortex movement, and pressure wave propagation characteristics during the whole process of combustion, the flame propagation mechanism of methane combustion at low initial temperature was established finally. Results indicate that five stages are divided during the methane combustion in a confined vessel: spherical flame propagation, "fingertip" shaped flame propagation, flame "skirt edge" contacts the side wall, "crescent" flame propagation, and typical "tulip" flame propagation. In the process of flame propagation, the reverse of the flame front and formation of the "tulip" flame can be immediately contributed to the interaction of the flame front, flame induced reverse flow, and vortex motion. However, the pressure wave propagation back and forth along the flame propagation direction has no obvious effect on the formation of tulip flame. When the distorted tulip flame is formed, vortex motion is not observed. The formation of the distorted tulip flame is caused by the superposition of the secondary pressure wave formed by the contact of the flame with side wall. However, because of the low intensity of pressure wave, RT instability is weak, and the distortion of flame front is not obvious. Flame propagation velocity and pressure wave are interacted with each other. In the process of combustion, the variation of flame propagation velocity and pressure rise rate show almost the same phase. The increase in flame propagation velocity directly leads to the increase in pressure rise rate, whereas the pressure wave propagation back and forth in the confined vessel leads to the oscillation of propagation velocity.