The distribution mode of arc current density determines the distribution modes o f both arc heat flux and arc pressure, which are the basis to understand the phy sical nature of welding arc and to establish the mathematical model of welding p rocesses. Based on the general principle of arc physics, a distribution model of current density on the deformed weldpool surface is established in this paper. Quantitative analysis is carried out to determine the influence of weldpool surf ace shape on the distribution of current density. The predicted results show tha t the distribution of current density is bimodal distribution near the arc cente rline, and that with increasing distance from the arc centerline the bimodal dis tribution is gradually diminished. The distribution mode of current density is m arkedly modified by the shape of weldpool surface. According to this distributio n model, the predicted weld geometry is in agreement with the measured one.