The paper aims at revealing the effect of blockage ratio (BR) on the flame acceleration process and the flame-vortex mechanism in an obstructed chamber based essentially on the experimental and numerical methods. In the experiments, high-speed video photography and pressure transducer are used to study the flame shape changes and pressure dynamics. In the numerical simulations, large eddy simulation (LES) with the flame surface density (FSD) model is applied to investigate the interaction between the moving flame and vortices induced by obstacle. The results demonstrate that the flame propagation process can be divided into four stages, namely spherical flame, finger-shaped flame, jet flame and volute flame for three obstacle BR configurations, and a small recirculation zone is observed above the obstacle only for BR = 0.5. The peak of flame tip speed and pressure growth rate increases with the blockage ratio. The generation and evolution of the vortex behind the obstacle can be attributed to the initial flame acceleration, while the subsequent flame deceleration is due to the flame-vortex interaction. Its addition, the transition from a "thin reaction zones" to a "broken reaction zones" is also observed in the simulation. (C) 2016 Elsevier Ltd. All rights reserved.