Abstract: In order to assess the ecosystem condition of Yantan reservoir， 3 points of different aquaculture types were set in Yantan reservoir to examine the main physical and chemical factors affecting water quality， sampling once a year. So as to construct the evaluation system of Plankton-index of biotic integrity（P-IBI） ， the grey relation analysis method was use to analyze the relationaship between P-IBI and water quality in Yantan reservoir. The results showed that， the ecosystem condition of pen-closing culturing pattern（point C）was better than that of cage culturing inferior intensive pattern（point B）and cage culture intensive（point A） pattern in Yantan reservoir. pH， water temperature， COD Mn were the major factors influencing P-IBI value.