Abstract: Based on the research of real-time measuring the weld pool width with a dual wavelength optical filter,a fuzzy control system of welding pool width has been developed.The structure of fuzzy controller and fuzzy variables are discussed in this paper. A fuzzy controller with two input variables and one output variable is considered to be suitable to control weld pool width.The input variables of the fuzzy controller are the variation of the welding pool width and its rate,and the output variable is the welding currert. In this paper, the designing process is described in detail. The testing result about the effect of welding current on the real-time measured welding pool width is used to build the fuzzy control relation and the control function forms.Based on these fuzzy control forms, weld pool width is easily controlled by adjusting welding current.