Abstract: Commonly conventional techniques are helpless to measure wall thickness of equipment in line. However, with the advantage of wide frequency spectrum, strong penetration and longer attenuation, pulsed eddy current(PEC) is very adaptable to measure wall thickness of metal in high or low temperature without insulation removal. The high-tech of PEC is becoming research focus of the world. As the practical insulation thickness different, so the lift-off between metal and probe is different. Researchers made many trials in order to know how lift-off influences measuring. Finally they found the relationship between lift-off and error, made quantitative analysis, established the formula of correcting data of inspection.