Based on COMSOL Multiphysics finite element simulation software, a numerical model for ultrasonic monitoring of double pulse resistance spot welding（RSW） is established,and the transient distribution of the ultrasonic field inside the workpiece during the spot welding process is studied. The built-in ultrasonic probe electrode is designed, and the ultrasonic online monitoring experiment during the RSW is carried out. The A-type and M-type ultrasonic echo diagrams characteristics are analyzed, the M-type echo diagram characteristic of typical RSW defects is studied, and a new method for identifying the fake welding based on the M-type echo diagram is proposed. The research results show that the RSW M-type diagram based on ultrasonic signal can reflect the welding process, and the fake welding spot can be quickly identified through the M-type diagram characteristic.