Abstract: By using a metallurgical microscope, a stereo microscope and a scanning electron microscope with an energy spectrum analyzer, the cracking defects of three kinds of common carbon steel after bending were analyzed synthetically.The results show that the main reason for cracking of the square tube in 90° cold bending is that the slag coiling occurs during the continuous casting process, it is then rolled under the skin of the plate, resulting in the discontinuity of the matrix.During welding of fire hydrant steel after 180° bending, cracking occurs. The main reason is that the temperature of the welding site is too high, the grain size is unusually large, and the reticular cubic cementite forms during cooling, which reduces the plasticity and toughness of the plate. Cracking of hot rolled plate in 180° cold bending is caused by the presence of aluminosilicate inclusion.