Abstract: Citricacid fermentntion process was studied in this paper.According to principle of the fermentation process kinetics,the mathematical models of biomass proliferation, medium consumption and citricacid production were given for the citric acid fermentation process. The calculation formula for mycelium concentration was derived indirectly from carbon balance,The parameters of the mathematical model were estimated by the GLS.The model curves from computer simulation were verified by the results of experiments.Satisfactory results were achieved by using the extended Kalman filtering iterative algorithm for on-line state estimation.