The invention relates to an ultrasonic automatic detection system of tubing with a variable wall thickness and step, and belongs to the detection technical field. The detection system comprises a track, a computer measurement and control system, a carrier roller and a carrier roller transmission device, wherein traveling wheels are arranged between the track and an automatic detection trolley, a fault detection water tank is arranged under the track, the carrier roller is arranged at the bottom of the detection water tank and connected with a reducer output shaft of the carrier roller transmission device, and the automatic detection trolley crosses on the track. According to the detection system, a water immersion ultrasonic method and a vortex method are adopted to automatically detect the tubing with the variable wall thickness and step, 100% detection is achieved through tubing rotation and linear movement of a probe along with a drive device, sound incident angles needed by detection is reached through drive and adjustment of the probe, and the computer technology is used for controlling a mechanical system, detection processes and data processing and output.