Welding has been widely used in industry for hundreds of years, and pursuing higher weld quality requires a better understanding of the welding process. The x-ray imaging technique is a powerful tool to in situ observe the inner characteristics of the melt pool in the welding process. Here, current progress in in situ x-ray imaging of the welding process is concluded, including the experiments based on the laboratory-based single x-ray imaging system, the laboratory-based double x-ray imaging system, and the synchrotron radiation tomography system. The corresponding experimental results with the in situ x-ray imaging technique about the formation and evolution of the keyhole, melt pool, pore, solidification crack, etc., have been introduced. A new understanding of welding based on the current progress in in situ x-ray imaging of additive manufacturing is concluded. In addition, the future development trend of applying x-ray imaging technology in the field of monitoring the welding process is proposed.