Abstract: In this paper, X80 high-strength pipeline steel is taken as the research object. Combined with numerical and theoretical analysis, the constraint parameters r_p0^1/2、r_p\max ^1/2、A_pz^1/4、\chi _P based on the geometric characteristics of the plastic zone at the crack tip are studied for the crack tip stress of different single edge notch tensile SE(T) specimens. The in-plane and out-of-plane constraint effects on the fracture toughness of SE(T) specimens of X80 pipeline steel were quantified by fracture toughness tests. The relationship between r_p0^1/2、r_p\max ^1/2、A_pz^1/4、\chi _P, the uniform of in-plane and out-of-plane constraint based on the plastic zone at the crack tip, and the stress intensity factor K were established. The analytical relationship between the constraint parameters and the T stress (T₁₁ and T₃₃) was determined, and the fracture toughness prediction methods under small-scale yield conditions were proposed through finite element simulation and theoretical analysis.