This study addresses the constitutive relationship of the welded coarse-grained heat-affected zone (CGHAZ) in 80-mm-thick DH36 marine steel plates. By integrating quasi-static tensile testing, digital image correlation (DIC) technology, and metallographic analysis, we systematically investigated the mechanical property differences and underlying mechanisms between the CGHAZ and base metal (BM). High-precision DIC technology enabled strain field characterization at the microscale in the CGHAZ, while the Ramberg-Osgood model was adopted to establish a dual-material constitutive equation. The results demonstrate that grain coarsening induced by welding thermal cycles significantly influenced the mechanical responses: the CGHAZ exhibited enhanced tensile strength but reduced plastic compatibility due to decreased grain boundary density. Notably, gradient differences in elastic modulus (CGHAZ: 184 GPa vs. BM: 213 GPa) and yield strength (CGHAZ: 363 MPa vs. BM: 373 MPa) between the BM and CGHAZ necessitate strict differentiation in engineering design. This work overcomes the limitations of oversimplified CGHAZ properties in conventional design approaches, providing a novel methodology for strength assessment and lightweight design of marine structures. The findings offer critical theoretical insights and practical guidelines for enhancing the reliability of offshore engineering equipment.