In this paper, a three-dimensional simulation model for laser-cladding processes of magnesium alloys is proposed. The applied loading is a moving heat source that depends on process parameters such as power density, laser beam diameter and scanning speed. The effects of process parameters on the melt pool are quantitatively discussed by numerical analysis. In these parameters, Marangoni force is the most important in affecting the molten metal flow and the contour of the melt pool. Both the length and depth of the melt pool vary sharply with temperature dependence of surface tension when the absolute value of this temperature dependence is at lower value.