A physical and mathematical model is presented on detection of pipeline wall thinning by infrared NDT,and a cal-culation method is given about how to compute the inner diameter of the pipeline wall through detection of the temperature distribution of pipeline surface. In order to improve the reliability of detection ,a safety coefficient is employed which is deduced from analyzing the error of temperature measurement of infrared imager. The conclusion can be drawn that the temperature difference on the outer surface of the pipeline can reflect the nice thinning of pipeline wall. The lower the thermal conductance or effective thermal conductance of pipeline wall and fluid in the pipeline ,the more seriously the pipeline wall is thinned when both the temperature of fluid and that of the wall are same.