Abstract: Based on the investigation data in May 2005, a GIS-based evaluation model was made, then the present status of the ecological environment quality in Daya Bay was accessed and the spatial distribution patterns of the quality were simulated. The results showed that the comprehensive quality index of the whole assessed sea area was 0.710.0568, which indicated the quality status was at the better level. There were three defects were exposed in this assessment: eutrophic level of the water and the phytoplankton abundance were higher, the phytoplankton diversity was lower. The simulation of the environment qualitys spatial distribution patterns showed the whole maritime space assessed could be divided to three regions according to the grade of ecological environment quality: the region with best quality, the region with better quality, and the region with fair quality, occupied 2.27%, 93.41% and 4.32% of the total area of waters assessed, the average comprehensive quality index were 0.800.0015, 0.710.0434, 0.540.0304, respectively.