When Q215 carbon steel strip was made of square tube, serious cracking occurred at the bending point. The cracking reason of Q215 steel pipe was analyzed by optical microscope and scanning electron microscope. The research results of inclusions and microstructure in three batches of experimental steel show that the main reason of steel pipe cracking is the existence of cementite which precipitates along the ferrite grain boundary. The cementite is hard brittle phase, which causes stress concentration at the grain boundary during bending deformation and initiates cracks; the secondary cause of cracking is the presence of large-scale sulfide inclusions more than 100 microns in the steel. By adjusting the process parameters of hot rolling and cooling, the amount of cementite in the steel is obviously reduced, the amount of pearlite is obviously increased, and the cracking rate of square tube is obviously reduced.