Abstract: An axisymmetric finite element model was established to simulate the nucleation process of sheet-tube single-sided spot welding(SSSW).The temperature distribution could be charted in electrode and workpieces, the nucleation process and nugget formation had been simulated by the incrementally coupled thermal-electrical-mechanical analysis.The great deformation of workpieces during sheet-to-tube SSSW makes the weld quality unreliable.Servo gun as a new electrode, which has some merits such as soft touch and variable electrode during welding process, is greatly suited to SSSW.Based on the characteristics of servo gun, the effect of variable electrode force on nugget formation was analyzed.The results indicat that the decrease of electrode force during welding process can increase the width of joining, improve the quality of weld with the same welding condition.The solutions can offer the theoretical basis for the wide use of SSSW in auto production.