Abstract: The finite element analysis software Simufact.welding with the double ellipsoid heat source was used and the numerical simulation of three-layer and three-pass welding process of the saddle weld between the box and pipe seat with riding header connection was carried out, and the law of temperature field, stress field in the welding process and postweld deformation was analyzed. The results show that the peak temperature of the second layer (filling layer) and the third layer(cover layer) is higher than that of the first layer (bottom layer), the maximum stress position appears at the saddle weld between the pipe seat and the box. After welding, the deformation of the box near weld is the largest, and the concave phenomenon occurs after welding cooling of the pipe seat.