Abstract: When the dynamic current lag angel as the quality information is used to monitor the nugget quality for resistance spot welding, the serious electrical noise must be produced while the welding heat percentage is being real-time adjusted in a wide range. If this electrical noise could not be real-time removed, it would be wrongly regarded as useful quality information. Therefore the feedback control effect of monitor and the quality of nuggets would become worse. Based on the heat linear control theory about single-phase alternating current welder for resistance spot welding, the twin-function has been set up in this paper, and then the twin-function table for control system has been set up too. These means laid the theoretical foundation of removing the electrical noise from the mixture information of the dynamic current lag angel in the nugget growing process of resistance spot welding.