Relative abundances of marine plankton Dictypochaeae and freshwater plankton Pediastrum were systematically determined from a drill core through Holocene sediments in Yodoe Plain, to decipher environmental changes in the Paleo-Yodoe Lake. Compared with the difficult and time-consuming diatom assemblage analysis normally used, the above method offers a simple but valuable method for the study of environmental changes in brackish lakes, as the two species concerned are easy to identify under the microscope due to their characteristic features. Chemical analyses of TOC, TN, C/N, TS and C/S were also made from the same core. It is well-known that total sulfer（TS）and carbon-nitrogen ratio（C/N）are sensitive indicators of freshwater-brackish-marine conditions. Environmental indicators from the chemistry are compatible with those derived from the plankton abundances.