Abstract: The temperature field of workpieces on the change of section and the non-change of section was simulated with the finite-difference approach in the arc welding.The process of dynamic heat output of the molten pool and the penetration variation were quantitatively analyzed.Moreover, by contrasting the results of numerical simulation on two kinds of workpieces,some conclusions were induced that the variation of heat output of pool and the penetration had homologous relation with the temperature gradient of length direction and width direction of HAZ and thus the penetration control on the energy equilibrium was based on these statements.