There exists a large of aquatic Myriophylhvn and Cyperaceae Pollen and Pediastrum algae fossils in Core Chao-2 in Dianchi, Comparing the pollen and algae fossils with the distribution of sporollen and algae in the surface of Dianchi Lake, it is clearly shown that the lake used to be a shallow lake with the water level shallower than that of the present stage about 40000-50000 yr.(B.P.). The lake basin extended gradually and the lake-level rose slowly up to the maximum of about 4.5m above the present one. But the lake became reduced about 28000yr.(B.P.). At the beginning of the Holocene, the basin began to shrink, the lake-level began to drop down, the whole lake basin was gradually filled up.