Abstract: It is difficult to diagnose some faults of satellites using the knowledge of only one symptom field. Based on Evidence Reasoning Theory, this paper introduces a new method to diagnose the faults. Information of different symptom field of satellite faults is merged by use the new method, thus the faults can be located more accurately. Firstly, the necessity of applying Evidence Reasoning Theory to merge the information in satellite fault diagnosis is analyzed. Secondly, the theoretical method of applying Evidence Reasoning Theory to merge information of different symptom field of satellite fault is studied. Thirdly, the practical application of the method is given. Fourthly, the notations that should be paid more attention to are analyzed. Finally, the conclusions of this paper are given.