A model of nitrogen and phosphorus dynamics in mesocosm experiments was established on the basis of a summary and synthesis of the existing models. The established model comprised seven state variables (DIN, PO4-P, DON, DOE phyto-plankton, zooplankton and detritus) and five modules-phytoplankton, zooplankton, dissolved inorganic nutrients, dissolved or-ganic nutrients and detritus. Comparison with the in situ experimental data in Laizhou Bay at the end of August 2002 showed that this model could properly simulate the variations of DIN, PO4-P, and phytoplankton biomass in a mesocosm pelagic ecosystem. It was found that not only the model structure but also the parameters adopted were fit for simulation. The sensitivity of the main state variables to the parameter change was assessed by sensitivity analysis. All these results are useful for studying the control mechanism of biogeochemical cycling of nutrients in Laizhou Bay and other Chinese coastal waters.