The invention discloses a solder joint inspection method for electric resistance welding based on a full focusing synthetic aperture technology. The method comprises the following steps: placing a phased array probe in a to-be-tested area, transmitting a pulsed electrical signal to each composite material wafer of the phased array probe by a pulse emission receiver, generating ultrasonic waves bya composite material wafer transducer, enabling the ultrasonic waves to generate reflected echo signals on a to-be-tested workpiece surface, a first metal layer bottom surface, a fusion zone bottom surface and the like, receiving the echo signals by the composite wafer transducer, feeding the signals into an amplifying circuit and an AD (Analog-Digital) conversion circuit in groups for performingsignal processing, feeding into a data processing composite unit, performing stack imaging on the echo signals by utilizing a full focusing synthetic aperture algorithm, performing three-dimensional imaging, displaying the image by a display module, obtaining the fusion zone size, and obtaining the nugget size by virtue of database comparison; and in addition, performing pseudo-color processing onthe obtained three-dimensional images, separating the solder side bottom layer from the fusion zone, recognizing the defect parts, and finally performing defect part display by the display module.