The effects of sponge blockage ratio on flame structure evolution and flame acceleration were experimentally investigated in an obstructed cross-section tube filled with stoichiometric hydrogen-air mixture. Experimental results show that the mechanisms responsible for flame acceleration can be in terms of the positive feedback of the unburned gas field generated ahead of the flame, the area change of the gap between the sponge and the tube, and the interaction between the flame and the shear layer appearing at the sponge left top corner. Especially, the last one dominates the flame acceleration and causes its speed to be sonic. Then both the second and third contribute to the violent flame acceleration. In addition, the unburned gas pockets can be found in both upstream and downstream regions of the sponge. With increasing blockage ratio, the unburned gas pockets disappear easier, and the flame acceleration is more pronounced. Moreover, the sponge tilts more evidently and resultantly the maximum tilt angle increases.