A mechanism model simulated density dynamics of the planktonic rotifer in Xinlin Bay Reservoir.The data were based on the samples taken at 2 months intervals from October and 1999 to August 2001.The factors included ciliate biomass, phytoplankton biomass, bacteria-density, temperature (T), salinity, dissolved oxygen(DO), water transparency(SD), chemical oxygen demand(COD) and the results showed that water transparency, salinity, ciliate biomass in south part of Xinlin Bay Reservoir were the dominant factors on controlling rotifer density, while phytoplankton biomass, bacteria-density, ciliate biomass, water transparency in north part of Xinlin Bay Reservoir were the dominant factors. By using multiple linear regression and non-linear regression to the model identification,two non-linear regression models were established.