Abstract: According to the requirements for corrosion detection of tank bottom plate, the identification principle of corrosion defect and the countermeasures to different influencing factors were analyzed based on the principle of ultrasonic pulse reflection method and common types of corrosion defects of tank bottom, and thereby, an immersion ultrasonic defect detection system with ARM and FPGA chips as the core was developed. As a defect detection unit of the in-service tank bottom inspection robot, the system comprises 16 5 MHz immersion probes, an ultrasonic signal processing board and an industrial computer. Therein, the ultrasonic probes and the ultrasonic signal processing board are connected by the 100 m long coaxial cables, which simplifies the explosion-proof requirements of the robotic circuits. The test results of mockup defect plates show that the system could find the defects with a diameter of not less than 5 mm and a corrosion depth of 10% or more of the original wall thickness, satisfying the requirements of practical applications.