Abstract: Based on the investigation of the relationship among the current,arc voltage and arc length in magnesium alloy sheets TIG welding,a new model based on the relationship among the current,arc voltage and arc length had been put forward which was used to control arc length exactly.At the same time,it combined the principle of PID and Fuzzy to design a kind of adaptive ALC (automatic length control) control system.The developed system not only overcame the limitation of traditional AVC (automatic voltage control) controlled system,but also resolved the problem of arc length control for the magnesium alloy TIG welding.The test results show that arc length could be traced and controlled according to the change of workpiece surface,and it also realizes arc length control well during welding initiation and termination with remarkably current change,moreover it has high dynamic response accuracy and good operation reliability.