Abstract: Formation reasons of cracks in H beam steel produced by semi-open way in a large H beam steel factory of Hebei were researched.26 specimens of representative crack in the plant were selected.The study shows that:the crack of stomach plank are mainly caused by inclusions in steel,and T[O] in steel is studied to find that the average T[O] in tundish is 94×10^-6 and the average T[O] in slab is 60.8×10^-6.Meanwhile,in contrast with the T[O] in 1 and 2 strem flow slab,it can be found that flow field distribution of the tundish is not reasonable.The existing tundish in the plant was simulated by FLUENT soft and it was found that the short dam of stabilized is lower in tundish,cannot completely eliminate short circuit flow,especially in the late stage of casting,short dam was used up not to work,and serious short circuit flow should be optimized.