Abstract: The heat exchanger in the acid environment is more prone to the problems of uniform corrosion, pitting corrosion, stress corrosion cracking and so on. In this paper, a multi-frequency eddy current testing technique is used to detect the middle pressure gas cooler in a natural gas processing station of China National Petroleum Corporation. The tube bundle corrosion is serious. It was found that the tube bundle was corroded seriously, and the proportion of tubes with wall thickness loss being greater than sixty percent accounts for more than seventy percent. At the same time, the veracity and reliability of the test results were verified by dissecting the pipes in the field.