Abstract: In order to improve the accuracy of fillet weld tracking of robot based rotating arc sensor, and to solve the problem that the horizontal stroke is not enough long when tracking weld based on horizontal slider. Combined filter was used to filter out most of the noise of welding current. Using least square method to fit the sampling point and then the slope of the fitting line was used to identify the deviation. The deviation and the sum of the last three times of deviation were taken as input parameter of fuzzy controller. Through fuzzy processing, fuzzy reasoning, single point set gravity method, the fuzzy controller was designed by clarifying the fuzzy parameter. The fillet weld was tracked based on differential movement of two rounds. Finally the fillet weld was experimentally tracked. The experiment result showed that the method solved the problem of insufficient length of horizontal stroke in tracking weld, and it also improved the reliability and accuracy of the rotating arc sensor in tracking fillet weld.