According to the characteristics of diesel engine sliding bearing vibration with nonlinear and unsteady, a new method was proposed for monitoring sliding bearing fault. The relationship was analyzed between the sliding friction bearing fault and the sliding bearing vibration signal during diesel engine combustion time. And the contact friction fault characteristics were established of the sliding bearing in diesel engine combustion time. Then a new feature value to judge plain bearing failure was created by the relationship between the sliding bearing and the two characteristics. The result shows that the method is simple and accurate, which provides a new approach for the diagnosis of sliding bearing friction fault.