Abstract: The fracture of 45 steel plate was analyzed by means of macrocosmic and microcosmic. The results showed that the fracture and crack of 45 steel plate was caused by internal stress when gas cutting; asymmetry heat make plate partly and cooling rate excessive rapidness wastcausing widmanstatten structure, and thrice cementite producing along the grain edge. All that badness microcosmic structure make nature of material became brittleness, and crack when bending.