The invention relates to automatic detection equipment and a positioning method for a weld joint at safe end of nuclear reactor pressure vessel. The automatic detection equipment comprises a bracket body, a scanning device and a positioning mechanism, wherein the positioning mechanism comprises a supersonic distance measurement sensor for detecting the distance between the front end of an end face strut and the wall of a cylinder, a horizontal inclinometer user for detecting the inclination angle of the detection equipment in X-axis direction, an axial approach switch for fixing the initial position of the scanning device in Y-axis direction and a circumferential approach switch for fixing the initial position of a supersonic probe frame in X-axis direction. After the rough positioning and the accurate positioning in axial direction and circumferential direction, scanning work can be performed. By the invention, the problem that scanning feedback data is not in correspondence with scanning positions, caused by probably existed axial or circumferential offset and the zero position offset of the supersonic probe frame in a pipe nozzle, can be overcome, thereby ensuring the readability of detection results. The whole process is controlled automatically and operated very conveniently; and thus, the invention has grater promotion and application values.