A field study was conducted in the coastal area of Caofeidian, Bohai Sea in October 2015. A broad seagrass bed covering approximately 10 km² was discovered in the northwestern area of Longdao Island. It is the largest seagrass bed discovered so far in Bohai Sea and Yellow Sea, China, with Zostera marina as the dominant species. Z. marina on the shoal showed patchy distribution, with coverage of 2.8±1.1% and shoot density ranging between 28.21±6.35-101.33±17.99 shoots·m^-2.The number of leaves per plant, canopy height, rhizome length and root length ranged between 2.00±0.00-4.70±0.64, 15.20±5.84-62.10±7.34 cm, 2.67±1.70-22.20±3.92 cm and 3.67±2.36-8.00±1.90 cm, respectively. The average seagrass biomass was 100.48±47.16 g DW·m^-2, and the total biomass was estimated to be 1.0×10⁶ kg (dry weight). A great diversity of nekton and benthic animals was observed during this survey, including a large quantity of juvenile and young fish and molluscs, as well as adult fish and shellfish species. However, the co-verage and shoot density of the seagrass were relatively low, and the number of branches and leaves per plant were also relatively few, which indicated a trend of serious decline in the seagrass bed, possibly attributing to seasonal changes as well as excessive anthropogenic activities. This is the first report on the species identification, distribution and ecological characteristics of seagrass in the Caofeidian area, providing an important basis to initiate further study for protecting the seagrass ecosystem in this region.