Abstract: The synthesizing control scheme is put forward based on the requilement of narrow-gap all-position welding technology in terms of the moving track of tungsten electrode. Soft touch-striking arc of the tungsten electrode has been made through researching the mechanism of parameters effect on tungsten track. Two projects of arc length adjusted by backgroud and pulse vohage or backgroud voltage have been obtained by analysing the track of swing tungsten electrode. Autonomously tracing system controlled by computer on transversely moving track has been designed in order to avoid the climbing arc phenomena on two side walls of narrow gap U groove.The changeable gain proportional integral differential controller of arc length, based on arc voltage sensor and the single chip microcomputer, is developed on the characteristic of the influence of gravity on tungsten electrode vertical moving track during all-position welding.