Distribution laws of blast loads on arch structure induced by underground explosions are studied by numerical method. The results show that the load distribution on arch is mainly effected by the explosion height

the arch span and the angle of monitoring point. The distribution form of load is shape of saddle as a symmetry axis around vault. Load distribution on the arch will be more uniform with the increase of the ratio of explosion distance and span. Conversely

it will be more concentrated. The load distribution on side-wall is mainly effected by the explosion height

the arch span and the side-wall height; and it reduces gradually from wall tip along the wall

finally tends to even distribution. On this basis

a method to calculating load on arch structure with large span is proposed. After calculating the load factor on arch structure

it is found that the load factor around the arch is not a constant value and increases with the increase of arch angle. The minimum value of the factor appears at the vault and it equals to 1.5.