Longitudinal crack on HRB335 hot-rolled ribbed bar surface in cold bending test in HISCO was ever a problem that caused great economic losses. To find the main reason, continuous cast product macro-acid etching test，metallographic examination，scanning electron microscope and EDAX are carried out. It is regarded that the defect is mainly caused by serious subsurface blowholes and internal cracks in the cast product，which can run through the surface in the later hot working process and are oxidized. As a result, the inner surface of these defects can not be welded-on in the rolling process and the cracks happen finally. Related measures are suggested to eliminate the defect.