Abstract: Described here in the essay is a system for on-line inspection of 3D surface flaw of the hot-rolled steel tube in cold state. Equipped with multi-channel 3D sensors,the on-line inspection system is capable of on-line 360° 3D imaging for the tube as going through,and thus carrying hi-accuracy identification of the 3D pit defects via setting the thresholds like defect depth and area,etc. When used for inspecting the cold-state surface quality of the hot-rolled steel tube,the said system can well make up for the shortcomings as staying with the commonly-used inspection methods like the contact-type ultrasonic detection,the leakage flux testing and the eddy current inspection. Moreover the system can also be used to perform 3D flaw inspection of the piercing plug bar.