Abstract: In order to study the temperature field and stress field of multi-layer multi-pass welding of 1Cr18Ni12austenitic stainless steel pipe Y-joint, the mathematical equation of irregular space ellipsoidal intersecting seam of Y-joint of two pipes with same size（inner diameter of 60 mm, outer diameter of 80 mm） was derived. A geometric model of the component was established, and after mesh division it was introduced into Simufact Welding software to carry out the welding simulation. The simulation used the classic Goldak double-ellipsoid heat source model with the same test’s working conditions was loaded, and the tracking points were set, the temperature field and stress field of the multi-layer multi-pass welding of the tube Y-joint were calculated. By comparing the experimental values, it is found that due to the steep temperature change, it produces thermal stress in the welding area, and the expansion and contraction of the metal in the welding area and its adjacent area leads to the appearance of compressive and tensile stresses. These stresses produce deformation. Compared with the branch pipe, the effect of welding stress on the main pipe is more significant, there is a bending deformation in the main pipe.