Abstract: A few kind of foreign trophic state indexes or models are briefly described and assessed in this paper.It is shown that these indexes or models are all statistical ones and merely are adapted to assess tropic state of lake in some research regions or another comparable regions.So a new kind of trophic state index is proposed.The index is of characteristics as follows: totality,simplicity and reliability of showing lake trophic states,authority of assessment standards,and equality among the sub-indexes.The index shortcoming is that it is only adapted for phosphorus-limiting lakes. In this paper the trophic state index is used to assess the trophic state of the West Lake in Hangzhou.The assessment result is both reasonable and reliable.