Abstract: Steel-glass fiber reinforced polymer (GFRP) co-cured adhesive joints are widely employed in wind turbine blade root connections, where their structural integrity is critical for ensuring wind turbine safety. Accurate measurement of Mode I fracture toughness serves as a prerequisite for investigating interfacial failure mechanisms in such bimaterial adhesive joints. To obtain pure Mode I interlaminar fracture toughness at the steel/GFRP interface using the double cantilever beam (DCB) method, a sandwich DCB specimen configuration was proposed for interfacial fracture toughness characterization. Firstly, theoretical frameworks for crack tip energy release rate calculation, including the virtual crack closure technique (VCCT) and J-integral method, were established. Finite element models were subsequently developed for three DCB configurations: uniform-thickness, equal-bending-stiffness, and sandwich specimens. Fracture behaviors under Mode I loading were numerically analyzed using VCCT-based finite element simulations, obtaining load-displacement curves, strain/stress distributions, and critical strain energy release rates at crack tips. Finally, comparative analyses were performed between numerical predictions and experimental measurements for sandwich DCB specimens. Key findings reveal that among the three configurations, the sandwich DCB specimen exhibits minimal strain deviation between upper and lower surfaces (<11%) and the lowest proportion of Mode II strain energy release rate at crack tips (≤0.05%), establishing it as a high-precision methodology for determining Mode I interlaminar fracture toughness in co-cured steel/GFRP interfaces. Results also demonstrate that the VCCT effectively simulates fracture behaviors of co-cured steel/GFRP interfaces, with simulated load-displacement curves showing strong agreement with experimental data.