Based on the theory and method of gray relational analysis, the similar coefficient was introduced to reduce the influent of relational degree affected by the maximum value or the minimum value in data and relational coefficient. The suction fan faults were diagnosed according to the synthetical relational degree composed by the relational degree and the similar coefficient.It is indicated that the method is available for the fault diagnosis of suction fan and has the characteristics of simplicity, low computation and high reliability.