Abstract: An ultrasonic automatic wall thickness measuring system of storage tank was introduced. A four-wheel driven mini-car acted as the carrier of the system, the mini-car was attracted on the wall by a piece of permanent-magnet built under the car, a rubber wheel ultrasonic probe was used to collect data, which need no couplant. When the car running, the system measured the wall thickness of the tank and stored the thickness data. When analyzing the thickness data, different filter bands were set according to the different limited layer thickness. The testing results of the system was proved to be in agreement with the testing results detected by ordinary wall thickness measuring method.