On the basis of the monitoring data of water quality during the period of 1990s, this paper presents the assessment of water quality in Honghu Lake by the grey-mode identification model. And the water quality comprehensive index isolines of Honghu Lake's different seasons are drawn by applying the surfer8.0 software. From the isolines, it can analyze the spatial distribution features of water quality in Honghu Lake. The research results indicate that the water quality of Honghu Lake remains between the grade 3-4 in criterion of surface water and in mesotrophic state. In autumn, the water quality of Honghu Lake is better, but in spring it is the worst. And in different seasons, it changes greatly. The difference of water quality between fish-screen regions and intake-outlet water areas is very obvious.