Abstract: The fault diagnosis for air pressure-flow control system of fermentation process was proposed. The system linear model was established, it contains some uncertainties consisting of linearization error and unknown disturbance. To the sensor faults, a new fault diagnosis method was presented. The mix estimation of the influence of the uncertainties and the fault was given using residual information, then the fault value was calculated. Compared with the old methods, the method not only guarantees robustness of the algorithm, but also enhances the sensitivity of fault estimation. A simulation example was used to illustrate the results.