Nine sampling sites along three sections in Lake Chenghai were established by Global Positioning System for a study on phosphorus forms in a period of a year. The characteristics of the distribution and variations of phosphorus forms in deep lake on plateau are analyzed. The results indicate that the total phosphorus content is between 0.008 and 0.155 mg/L, and the annual average content is 0.046 mg/L. The percent of existent state of phosphorus forms is 54.35% particulate total phosphorus, 19.56% dissolved organic phosphorus, 26.09% dissolved inorganic phosphorus. The characteristics of the distributions and variations of phosphorus forms are closely related to vital processes and cycles of phytoplankton. The horizontal distribution pattern is easily affected by the composites effects of biological processes of plankton, and movement and feeding of lake water. The distribution of phosphorus is relatively homogenous from the surface layer to the sub-bottom layer, and increase with various degrees at the bottom layer.