Abstract: In order to grasp the actual state and change of the point defects on the aluminum pool bottom of 49-2 swimming pool reactor (49-2 reactor for short), to further provide data support for the service performance evaluation, and to ensure the safe and stable operation, a water immersion ultrasonic measurement system was developed according to the information of the pool bottom structure, the media in the pool, and the pool wall materials. The distribution of point defects at the bottom of the pool is examined comprehensively, and the point defect depth is accurately measured. The water immersion ultrasonic measurement equipment integrates a water immersion ultrasonic transmitter, a multi-axis scanning device and a controllable underwater floating robot. It uses ultrasonic waves to scan uneven areas such as pits on the surface of the aluminum plate. Due to changes in the surface properties of the material being tested, the sound path of the echo reflected from the ultrasonic interface changes, and the depth of the pit is obtained by measuring the difference in the sound path of the echo sound beam on the intact surface and the bottom of the pit. Before the on-site measurement, a 15 MHz water immersion point focusing probe was used to scan the pit simulation test block. At the same time, a laser confocal microscope (8.0 μm step diameter) was used to measure the pit simulation test block. The scan data were compared and analyzed to determine Measurement accuracy was confirmed. In the actual inspection, the remote carrier device, that is, the underwater floating robot is used to move the scanning device to the inspection area, grid lines are used for grid scanning, and signal collection and analysis are completed through a multi-channel ultrasonic instrument. Finally, the accelerated corrosion experiment simulation analysis results were used, combined with the maximum point defect depth data measured in this study for comparative analysis, and the time-limited aging analysis conclusion of the 49-2 reactor bottom material was given. This study is the first time to conduct remote underwater nondestructive testing of the actual state of the defects at the bottom of the pool in 49-2 reactor. Compared with the historical data, the distribution of the defects at the bottom of the pool is basically consistent with the measured data of the pit depth, and the maximum pit depth is 1.495 mm. The research conclusion provides important data support for time-limited aging analysis of materials at the bottom of 49-2 reactor pool, and the underwater ultrasonic measurement technology developed can also be applied to aging management and service life analysis and demonstration of the same type of pool reactor vessels and spent fuel storage pools.