For the sustainable development and utilization of marine resources, and protection of marine ecological environment, coastal countries have successively implemented marine spatial planning (MSP). MSP is considered as an important marine management tool, which is an indispensable part of marine ecological civilization construction in China. Scientific and reasonable division of marine spatial units is the basic work of marine spatial monitoring, evaluation, planning and management, which is of great practical significance for the protection of marine ecological environment and the sustainable development of marine resources. However, the characteristics of the ocean, such as mobility, openness and boundary ambiguity, make the division of marine spatial units more complex and difficult than that of land. In this study, we collect a large number of data from environmental monitoring sites in Dongshan Bay. Based on the comprehensive and detailed data which can capture ecological environment status, we propose a technical scheme of spatial evaluation units division which is suitable for fine-scale sea areas. The scheme constructs the index system for the division of marine spatial evaluation units based on the physical, biological and chemical elements. Combined with GIS technology such as interpolation and spatial overlay analysis, Dongshan Bay is divided into 67 basic spatial evaluation units, which is a more accurate and scientific division of marine spatial units. On the basis of marine spatial units, local government can carry out the marine spatial monitoring, evaluation, planning and management. The marine environmental protection strategies and marine exploitation activities can be formulated for the sea areas with different environmental endowments by considering the local conditions. This division approach can not only provide technical support for the MSP of Dongshan Bay, but also can be a supplement of spatial units division approach for marine spatial monitoring, evaluation, planning and management. This division approach is suitable for fine-scale sea areas, but it is also applicable to large and medium-scale sea areas where complete and detailed environmental monitoring data can be obtained. The technical scheme of marine spatial units division proposed in this study has a wide application prospect.