Abstract: Intergranular corrosion is a critical degradation mode for copper and copper alloys in marine engineering applications, yet it lacks sufficient quantitative characterization. This study investigates the intergranular corrosion behavior of pure copper in a simulated seawater environment using a quasi-in situ observation method. Cross-sectional samples containing high-angle grain boundaries (29°, 40°, and 50°) and two types of twin boundaries (coherent and incoherent) were prepared via focused ion beam (FIB) technology, followed by quantitative analysis of intergranular corrosion depth. The results indicate that high-angle grain boundaries exhibit preferential corrosion characteristics during the corrosion process, and the corrosion depth is negatively correlated with the grain boundary energy. The 50° grain boundary possesses the highest grain boundary energy and shows the strongest intergranular corrosion, reaching a depth of 7.7 μm after 5 days of immersion corrosion. Coherent twin boundaries exhibit superior corrosion resistance due to their highly ordered atomic structure and extremely low grain boundary energy. In contrast, incoherent twin boundaries show significant corrosion susceptibility owing to their deviation from the low-energy 111 crystallographic plane. Furthermore, the intergranular corrosion rate is directly correlated with the atomic packing density of the grain planes adjacent to the grain boundary.