To realize testing of surface crack defect in X80 pipeline, an entirely non-contact and non-destructive detection method of using pulsed laser to excite ultrasound and using continuous laser to receive ultrasound is proposed, and laser ultrasonic testing system is built. By doing the A-scan and B-scan of pipe surface, adopting amplitude normalization, filtering, differential and other methods of signal processing to extract wave signal, propagation characters of ultrasonic surface wave in the pipe are explored. The result show that the reflection occurs in a part of the ultrasonic surface wave when encountering crack defect, and another part propagates along defect edge to the crack bottom, generating mode conversion and changing into transverse wave to propagate to the pipe inwall. Then, it is reflected back again to the crack bottom, generating again the mode conversion and changing into surface wave to propagate along the defect edge. Defect characteristics can be obviously identified, the maximum error of location and depth is 4%, and the resolution is 0.01 mm. Experiment results show that the laser ultrasonic testing system can effectively detect surface crack in the pipe.