Abstract: Described here in the essay are the composition,the measurement principle and the function of the isotope on-line measurement system. Also analyzed are the factors as affecting the thickness-measurement accuracy,and relevant actual application practice of the said system as made at the seamless steel pipe production site. The thickness measurement system is capable of performing thickness measurement timely and fully. It can not only judge the steel tube wall thickness accuracy,but also indicate the problems as staying with the production links. However since the system presents the wall thickness accuracy with the wall thickness of the sampling point,and there exists the out-of-tolerance risk for the wall thickness in the position between the radioactive sources,it should be avoided to substitute the displayed wall thickness for the full-length wall thickness of the pipe body,which leads to misjudgment of the actual wall thickness.