Abstract: The arc length is one of the most sensitive parameters that influence welding quality at all-position TIG welding. Due to the influence of tungsten electrode burning out, fromer pass appearance of weld, molten pool movement, weldment geometry shape and effect of gravitation on arc length adjustment at different position, if arc length adjustment were not adopted, the arc length could not be keeped fixed length. It is more extrusive that the force of gravity influences arc length at all-position welding, the some especial controlling measures must be taken in order to overcome its influence. Therefore, a Changeable Gain PID controller is developed lased on the single chip microcom puter controlling, in which the techniques of fast control, separated integral and appended non-sensitive scope are used for improving control properties.