With the progress of society and the development of the economy, people’s awareness of environmental protection is also becoming stronger. Therefore, the damage caused by marine oil spills caused by offshore oil extraction to marine ecology is also receiving more and more attention from people. Marine ecological damage assessment involves various disciplines such as environmental chemistry, biology, toxicology, and hydrodynamics, and requires a large amount of data support, making it a challenge in the field of marine claims. To quantitatively evaluate marine oil spill accidents, the Spill Impact Model Application Package model is used as the system framework to build a marine ecological damage assessment model. The MIKE21 model is used for physical fate simulation, while the biological effects model is used for resource damage assessment of the ecological environment and aquatic organisms. The model is validated using the oil spill accident in Dongshan Bay as an example. The validation results show that the total loss of aquatic organisms under conditions 1 and 2 is 67.30 tons and 56.03 tons, respectively. Under condition 1, the ecological environment of coral and mangroves is affected by an area of 8.805km2 and 5.859km2, respectively, resulting in a loss of 19.5168 million yuan in ecological resources. Under condition 2, the ecological environment of coral and mangroves is affected by an area of 7.646 km2 and 13.515 km2, respectively, resulting in a loss of 26.322 million yuan in ecological resources. In summary, it can be seen that the research on marine ecological damage assessment models for offshore oil extraction provides certain reference value for the field of marine ecological damage assessment in China.