The invention discloses an on-line detection method of continuous casting billet surface quality. According to the on-line detection method, a laser light source is taken as lightening equipment, an industrial-grade high-precision camera is used as image acquisition equipment, and a corresponding optical filter is arranged on the camera; when the surface of the continuous casting billet is exposed in laser beams, absorption, reflection and refraction features of oxide skin, cracks, recesses, scratches and slag pockets on the laser beams are different, and the difference is collected by the camera, and is shown in forms of imagines; and then defects observed in the imagines are positioned, sorted, preserved and underlined via defect detection algorithm. According to the on-line detection method, the laser light source with high power and the industrial linear array camera are combined, so that technological problems of continuous casting billet surface detection site imagine acquisition are solved; a distributed structure is used for data transmission, storage and processing, system stability is high, and storage and processing speed is fast; the imagine processing algorithm is firmly based on the features of detected objects, so that detection accuracy is high.