Abstract: The surface "triangular mouth" crack in CM690 anchor chain steel was discovered during rolling process. The defects were analyzed from four aspects: chemical composition, defect morphology, metallography and energy spectrum. The smelting and rolling processes were also checked. It is found that the content of titanium in the chemical composition of steel is relatively low, and frequent adjustment of drawing speed and high superheat in casting process. It is concluded that the "triangle mouth " crack on the steel surface is mainly caused by the transverse crack of the billet surface through rolling deformation. The nitride phase diagram of CM690 steel and the temperature change of different content of nitride were calculated by using ThermoCalc software. A crack mechanism of "triangle mouth" on CM690 steel surface was studied. The results show that the crack is due to the straightening temperature is just in the Al N nitride precipitation temperature range,Al N the precipitation at the grain boundary weakens the grain boundary binding force, and the external straightening force finally leads to the transverse crack. After the process optimization, the surface "triangle mouth" crack did not appear again.