Abstract: CCD is used as a sensor to detect automatically the laser interference fringes of arc plasma. The continuity and automatization of plasma interferometry process including photogrammetry, measuring and accounting have been realized. The diagnosing time of plasma is greatly shortened, and the prerequisite of real time control with microcomputer of welding arc is thus provided. This paper describes the basic structure and principle of CCD and presents the method of using "0,1" codes for transforming and dectecting fringes. The electric scheme of the system is also given. The test results of welding arc show that this method is feasible.