Constraint is the resistance of a specimen or structure against plastic deformation that contains geometric and material costraints. Both can affect the fracture behavior of a material significantly. For a material constraint, most studies focused on the strength mismatch of both sides of a crack, such as over-match and under-match. Nevertheless, the effect range of the material constraint also needs to be clarified. In previous studies, the effect range of a material constraint was demonstrated in different specimens. In this study, the actual and simplified nuclear safe end structures were selected. The J-M curves (where J is the J-integral, which reflects the degree of stress and strain concentration at the crack tip due to a wide range of yield; M is the bending moment), the areas surrounded by the equivalent plastic strain (PEEQ) isoline, and the failure assessment curves of the two structures under different material constraints were calculated to determine the effect range of material constraint in structure. The results show that the effect range of a material constraint exists in actual and simplified nuclear safe end structures. The J-M curves, the areas surrounded by the PEEQ isoline, and the failure assessment curves were unaffected by the material located out of the effect range. Compared to the actual nuclear safe end structure, the simplified structure had a lower geometric constraint, a larger material constraint effect range, and a higher failure assessment curve, possibly producing a non-conservative assessment result. Thus, in the design and structure integrity assessment of a nuclear safety end structure and other strength-mismatched structures, the influence of the material constraint effect range should be considered, particularly in the following two aspects. The first aspect is that in the design process, a material with weak properties should be designed out of the material constraint effect range. This can effectively avoid weakening of the structural properties caused by the weaker material. The second is that in the assessment process, the material out of the material constraint effect range does not need to be taken into account. Only the material in the material constraint effect range should be considered, which will reduce the difficulty and workload of an assessment.