Abstract: In order to investigate the defects of the bottom plate of a 20 000 m³ vertical product oil storage tank which has been running smoothly for 12 years, the pressure is loaded and 12 sensors are installed on the bottom wall plate of the tank. The acoustic emission data of the bottom plate of the tank are collected and analyzed to determine the corrosion point coordinates of the bottom plate. According to the detection results, the corrosion state of the bottom plate of the tank is evaluated. After tank cleaning and sandblasting, the size and relative coordinates as well as the shape characteristics of corrosion defects on the tank bottom plate were checked by opening the tank, and the specific causes of corrosion defects were further analyzed based on the construction and operation background of the tank. The investigation results show that there are both construction damage defects and pitting corrosion defects in the tank bottom, but all the known defect depths are within the acceptable range. The corrosion defects of tank bottom plate found in open tank inspection are all marked by acoustic emission testing, which proves the applicability and accuracy of acoustic emission testing technology in the detection of product oil storage tank.