The invention relates to a method for determining constitutive parameters of a butt welded pipe welding seam in the biaxial stress state, which comprises the following steps of: determining the constitutive parameters of the parent metal in the uniaxial stress state, thereby determining the range of the parent metal strength factor K<sim> in the perforated sample tensile finite element analogy, and the range of the hardening index n<sim>; and by determining the variation ranges of the strength factor and the hardening index, obtaining the relationship between different constitutive parameter combinations (K<sim>, n<sim>) and the combinations (F<sim>, Delta l<sim>) of tensile force and gauge length elongation, and establishing a recessive mapping relationship between the combination (F, Delta l) of tensile force and gauge length elongation and the constitutive parameter combination (K, n), thereby obtaining the constitutive parameters of the welding seam in the biaxial stress state. The method induces the planar biaxial tension stress state, which is more approximate to the true stress state of pipe deformation, in the uniaxial tension process of a perforated sample, can be used for accurately and efficiently determining the constitutive parameters of the welding seam ductility of a butt welded pipe, and is beneficial to enhancing the forming quality and the forming limit of the welded pipe.