Abstract: Based on the pyrolysis experiments of typical aquatic biomass in Poyang Lake with tube furnace, the precipitation characteristics of alkali metal were studied. The results show that the highest releasing percentage of sedge and eichhornia crassipes during pyrolysis is more than 60 % for K and Na. K mainly exists with inorganic salt formation in eichhornia crassipes, and precipitates with inorganic salt formation in the middle and later stage of the pyrolysis reaction, Na precipitation rate was higher than K, which is mainly due to interfacial bond energy of Na lower than K. Meanwhile, aquatic biomass species are important factors for K and Na releasing ratio during pyrolysis, which is related with the combined forms of K and Na in aquatic biomass and sample structure. Dynamical model was finally constructed based on the K and Na releasing behavior of aquatic biomass during pyrolysis. Using the model, the K and Na releasing characteristics of the sedge and eichhornia crassipes pyrolysis processes were analyzed, and the K and Na releasing characteristics predicted by this model are more satisfying in depicting the experimental data. It indicates that dynamical model can be applied to predict the K and Na releasing characteristics of aquatic biomass during pyrolysis.