Abstract: The technique of fault diagnosis based on model identification is beneficial for the quantitative determination of severity of faults. However, the inadequate theory of multiple fault identification needs further investigation. From the viewpoint of model identifiabilty, the relevant theoretical methods are discussed since the simultaneous faults will bring about unidentifiable problem of models. Based on the Fisher information matrix, an investigation method of model identifiability is proposed, which improves the model identifiability. The identification of the mass unbalance and initial bend of an experimental rotor is studied to illustrate the proposed method.