To achieve carbon neutrality goals, offshore wind power combined with a hydrogen energy storage system (OWP-HESS) is critical for integrating intermittent renewables. This study applied a “cradle-to-grave” process-based life cycle assessment (PLCA) to evaluate a 77.4 MW offshore wind farm coupled with a 45.0 MW electrolysis cell system, covering manufacture, transportation, construction, operation and maintenance, and decommissioning phases. It focuses on two hydrogen production routes, alkaline electrolysis (AEL) and proton exchange membrane (PEM), and covers 12 environmental indicators. Moreover, considering optimal economic efficiency, to adapt to the characteristic of “electricity–hydrogen cogeneration”, as well as to facilitate reflecting the efficiency differences between the two electrolysis technologies, the functional unit is defined as “0.4 kWh green electricity + corresponding green hydrogen”. Results show that offshore wind’s environmental impacts mainly come from manufacture (79.00%, driven by concrete/steel), while hydrogen storage impacts focus on operation/maintenance (66.03% for AEL and 96.61% for PEM, driven by electricity). PEM’s green hydrogen global warming potential (GWP) (0.96 kg CO₂-eq/kg) is much lower than AEL’s (1.81 kg CO₂-eq/kg) and China’s fossil-based hydrogen (≈40 kg CO₂-eq/kg). With an initial system lifespan of 25 years, a wind farm capacity factor of 41.30%, and a hydrogen production efficiency of 68.72% (AEL) and 69.89% (PEM), extending system lifespan by 5 years, raising wind farm capacity factor to 43%, and enhancing hydrogen production efficiency to 71% reduce emissions by 16.67%, 4.00%, and 2.16%, respectively. This study clarifies OWP-HESS’s environmental characteristics, confirms PEM’s low-carbon advantage, and provides support for its sustainable development.