The propagation of hydrogen/air premixed flame in channels were investigated, giving particular focus on the effect of aspect ratio through both experimental and numerical investigations. The schlieren system and pressure sensor were used to capture flame shapes and pressure changes, respectively in a channel with aspect ratio of 12. The phenomena of flame shape evolutions, including classic tulip inversion, featured T-shape flame, elongated tulip flame and tulip distortion captured in the experiments, were well reproduced by the numerical simulation. The predicted flame front speed, pressure build-up and their fluctuations are also in reasonably good agreement with the measurements. The validated numerical modelling approach was then applied to investigate the effect of channel aspect ratio on flame shape change, flame propagation and overpressure dynamics. It was found that with increasing aspect ratio, the flame shape changes become more vigorous accompanied by the amplified flame and pressured dynamics due to the enhanced pressure waves, flame-pressure wave interaction and flame instability. The Bychkov's analytical model was used to give predictions on the characteristic times of flame deformation in the channels. By comparison, the analytical model performs better for channel with lower aspect ratio.