Welds on fuel rods necessary for nuclear power generation have the highest possibility of leaking during combustion, and accordingly these welds must have integrity and strength. Welding methods of nuclear fuel rods commonly used by nuclear fuel manufacturers include GTAW, EBW, LBW, resistance upset butt welding, etc. In Korea, the resistance upset butt welding method is used. While the weld integrity is evaluated by fracture tests, there is a lack of understanding regarding the weld mechanism during the welding process. In this study, a coupled axisymmetric finite element model (FEM) was formulated to simulate the resistance upset butt welding process between the end plug and cladding tube by a numerical simulation. By simulating this process, the electrode displacement, dynamic temperature curves, melted volume, and resistance of the welding area were studied by changing weld parameters and validated through comparison with experimental results. Finally, in order to improve the integrity of weldment of the fuel rods in nuclear power plants, an estimation equation by regression model for optimal welding conditions was derived.