Abstract: The formation causes of longitudinal cracks on the surface of Q235 steel plate were analyzed by macrographic examination, chemical composition analysis, metallographic examination, scanning electron microscopy and energy spectrum analysis. The results show that: the decarburization phenomenon existed in the structure around cracks, and particle inclusions existed in the matrix around cracks. Besides the iron oxides inside cracks, secondary oxides of silicon and manganese were found in the matrix as well as sodium element. It was proved that the cracks were generated in the continuous casting crystallizer and eventually formed on the surface of the steel plate.