Abstract: K-TIG was used to weld 15-5PH plate. Tracing technique was used to anlysis K-TIG molten pool flow behavior. The welding beam consisted of three parts: the upper part was the marangoni convection circle produced by surface tension, the lower part was the convective circle produced by the interaction of surface tension and lorentz force, and the transition zone between the upper and lower convection circles. The trace method indicated that the W elements in the upper and lower parts of the molten pool were distributed in an arc shape around the convective zone, but the W element was no obvious shape in the transition zone. EDS analysis was carried out on the flow ring under the molten pool, and the results showed that the molten metal under the molten pool flowed by internal rotation. The results of calculation and test show that the upper and lower parts of the molten pool flowed violently, but the flow velocity of the middle layer was small. With the increase of welding heat input, the flow velocity of the middle layer had a tendency to decrease, and the grain size of the intermediate layer tended to grow.