Abstract: Based on theories of computational fluid dynamics and heat transfer, an axial symmetry finite element model of velocity and temperature field of spot welding for aluminum alloy is established. In the model, many factors are considered including both convection heat transfer inside moving boundary layer and heat conduction of solid metal, material property and contact resistance which are the function of temperature, the effect of the loss of heat by convection, radiation and conduction through spherical electrode, and latent heat of phase transformation. The model is discretized with finite element method. The calculated results show that the prominent convection cell is observed at the center of the molten pool and the maximum value of flow velocity is 0.1 mm/s. With direct current power supply, fusion nugget starts to generate within 5ms and expands rapidly along axial and radial directions. Circumfluence velocity vector delivers energy from fusion nugget center to nugget fringe by convection, which reduces temperature gradient inside nugget and drives nugget to grow. The calculated results agree well with measured value.