The invention relates to an online real-time X-ray detection system for welded tubes. The system is characterized by comprising a support, a base, an X-ray unit, a clamping device, an imaging device and a drive device; the base is slidably disposed on the support; the X-ray unit is disposed on the base; the clamping device is used for clamping a welded tube; the drive device drives the base to return. The system has the advantages that the X-ray unit and the imaging device are enabled to move with the welded tube, the welded tube is detected and photographed in the relative fixed state, imaging quality is good, 100% online detection of the length is achieved for the welded tube, the requirement for the lossless detection standard of the welded tube is fully met and the system is simple in structure and convenient to operate.