In order to reveal flame propagation behaviour and the mechanism of premixed methane-hydrogen/air mixtures (PMHAM), a pressure recording device was used for testing explosion pressure with different volume fractions of methane-hydrogen in a closed duct. Results showed that the explosion pressure and pressure rise rate increased with volume fraction up to an optimum fraction of 10 % for a maximum explosion pressure (P-max) of 0.60 MPa and a pressure rise rate ((dP/dt)(max)) of 54.93 MPa/s. Meanwhile, a high-speed video camera was used for testing flame propagation behaviour with the optimum fraction. Results showed that the flame propagation process consisted four dynamic stages-spherical flame, finger-shape flame, flame touching sidewalls, and tulip flame. Comparison of the numerical and experimental results showed reasonable qualitative and quantitative agreement. Additionally, the numerical simulation was used for addressing technical difficulties of understanding the mechanism of flame propagation. The simulated results confirmed the existence of two large-scale vortices nearby sidewalls generated by the gas-flow reflection and back-streaming; these increased vortices made speed of the flame front touching sidewalls exceed that of the middle, which ultimately resulted in reversal of flame front. This meant that the interactions of gas-flow reflection, back-streaming, and vortex motions were the immediate reason of tulip flame.