Abstract: Data sources of most traditional seawater quality evaluation are very single, so the evaluation results are non-comparative, but this paper will introduce multi-source monitoring data (buoy data, underway data and remote sensing data, etc.) into seawater quality evaluation. Use of multi-source monitoring data can increase the relative property of evaluation results in space and time scales,based on ArcGIS Engine, SQL Server database and Visual C# programming language and combined with GIS technology to build the seawater quality spatial evaluation system. The system will achieve multi-source monitoring data input and management, selection and inquiry of seawater environmental information, seawater quality spatial evaluation and cartographic output publishing of the comprehensive evaluation results, while it will also achieve the intuitive and visual expression of the seawater quality evaluation. Practice shows that the multi-source monitoring data used to the seawater quality spatial evaluation system is entirely feasible, and also provide a scientific basis for the control of environmental pollution, bathing placeand holiday resort planning.