A series of experiments was performed in an obstructed rectangular tube to investigate the effect of the blockage ratio of obstacle on vented hydrogen-air explosion. Internal and external pressure evolutions were monitored by pressure sensors while internal and external flow fields were recorded by high-speed schlieren photography. The overpressure histories, dynamic characteristics of external combustible cloud and flame behavior inside and outside the tube were discussed. The results show that the obstacle blockage ratio has a significant effect on the peak overpressure induced by the obstacle, but it has an ignorable effect on the peak overpressure resulting from the vent rupture. The peak overpressure induced by the obstacle increases with the blockage ratio. For the relatively lower blockage ratio of 7.14% and 21.43%, the peak overpressure is mainly induced by the enhancement of turbulence and burning rate caused by the obstacle. For the higher blockage ratio of 35.71%, 50% and 64.29%, the peak overpressure results from the combined effect of the flame acceleration and external explosion.