Abstract: In order to study the effect of surface tension on the flow behavior of molten pool and the morphology of molten pool, the active agent test was designed, and based on the finite element calculation software Fluent, the effect of surface tension on the flow behavior of molten pool was analyzed. The results show that when the active agent is not added, the surface tension temperature coefficient is negative, the molten metal on the surface flows from the near keyhole to the edge of the molten pool, welds are wide and shallow. After adding the active agent, temperature coefficient of surface tension is positive, so that the vortex reverse, forming a narrow and deep weld. With the increase of the absolute value of the surface tension temperature coefficient, the surface tension becomes larger and the fluid velocity increases.