The invention discloses a special composite non-destructive testing device for a laser welding joint. The special composite non-destructive testing device comprises an X-ray source and a flat panel detector, wherein the X-ray source is located above the welding seam zone of a welding seam work piece; the flat panel detector is located under the welding seam zone; and a supersonic phased array probe is also arranged on the welding seam zone. The invention also discloses a detecting method of the special composite non-destructive testing device. According to the method, the same location of one welding member is detected by a composite detection method including X-ray detection and supersonic phased array detection, the same position is analyzed by two kinds of data of the two detection methods, and the two kinds of data are mutually evidenced, so that the problems existing in the prior art that the specific position of a defect in a three-dimensional space of the welding seam cannot be positioned, and detecting and imaging effects are poor are solved; the positioning accuracy is higher; the accuracy is better; the detecting speed is high; and the positioning and the quantitative detection of a laser welding defect in the three-dimensional space can be realized.