Abstract: After hydrogen-induced cracking test,many hydrogen blisters appeared in the heat-affected zone of the L360MCS acicular ferrite steels. The caps of the hydrogen blisters were torn off by sticking a layer of high silver brazing alloys and extending. Then the fracture surfaces of the blisters were analyzed by SEM and EDS. The cell segregation inclusion of alloying elements containing Ca and Al sulfides and oxides were observed in most of the initiating zones of the blisters. The results show that internal stress and increasing vacancy concentration are the main reason of forming void in the inclusion zone. The key reason how high hydrogen pressure forms is that the hydrogen atom concentration in the void is lower than that in the basal body due to combination from hydrogen atoms in the void.