[en] Highlights: • A full-scale experiment system was established to investigate the manhole explosion. • The effects of methane concentration, ignition location and cover weight on the peak overpressure were studied. • The results showed that there were two peaks in overpressure histories caused by manhole explosion. • When the methane concentration was 10%, the peak overpressure generated by explosion was the largest. • The peak overpressure increased with the depth of ignition position and the weight of manhole cover. -- Abstract: Gas explosion in manhole often occurs in cities. Many previous researches on gas explosion are not suitable for manhole explosion because of the particularity of manhole structure. To investigate the gas explosion in manhole, a full-scale manhole model was established, in which the explosion overpressure of methane/air mixtures were studied experimentally. The variation of blast wave overpressure with time at different distances was analyzed. In addition, the effects of methane concentration, ignition location and manhole cover weight on the external overpressure after manhole explosion were obtained. The results showed that at the experimental conditions in this paper, under the influence of vent mode and flame propagation, the maximum peak overpressure caused by manhole explosion was mostly at the third measuring point. And there were two peaks in the overpressure histories. It was also found that when the methane concentration was close to stoichiometric ratio, the ignition location was further away from the manhole head, and the weight of manhole cover increased, the peak overpressure of blast wave caused by explosion increased. Besides, some suggestions were put forward for the risk control of manhole explosion accident based on the experimental results.