The sustainable development of marine economy involves many factors such as society, economy, resources and environment. It is almost impossible to comprehensively solve all problems in the system in one study. The potential of China's marine resources is huge, and the marine economy has maintained a rapid growth rate, but at the same time as rapid economic growth, there are also problems such as waste of resources, environmental pollution, and predatory management. The purpose of this article is to avoid the possible disorder in marine development, and to ensure the coordination and unification of marine economy, marine resource use, and marine environmental protection. In this regard, from the study of the offshore ecosystem, this paper establishes a system dynamic model for the sustainable development of the offshore economy. Through theoretical analysis, methodological discussion and case application, a more comprehensive study of the external value assessment of marine ecosystems is conducted. On the whole, the survey results show that people have paid more and more attention to environmental quality, living environment and the ability to supply resources, which are in line with actual conditions. The weight of all evaluation indicators, where CR is less than 0.05, shows that the distribution of weight is reasonable. The results of the study show that human-dominated value in marine ecosystems accounted for 56.1 %, aesthetic entertainment tourism value accounted for 13.9 %, and ecosystem-based values accounted for 43.9 %.