Abstract: Electrogas arc welding (EGW) is an important method for vertical position welding;the quality of welding product is highly depended on its welding arc length stabiliy, so there is a necessity of arc length control for EGW.Based on crawl type arc-welding robot, this article studies the variation of the arc length during the welding, and brings up a two-level linkage arc length control system composed by a slider and the welling vehicle.The advantage of the slider in dynamic response gives the system a good performance of fast and high precision arc length control.By controlling the vehicle travel speed, the slider always stay s in its neutral position, which makes the system maintains the ability of widest range adjustment.This system has an wide-range adjustment, high dynamic response and high precision as well.