For analyzing the vibration characteristics of a kind of structure with clearances,and for discussing the rationality of prekeeping clearances in protecting the special substructure, we reduce the nonlinear structure with clearances to 1DOF system with clearances and 2DOF system with clearances,by adopting the mass and the firstorder bending stiffness of its substructures on the basis of Assumption Modal Method and its vibration results. We analyze the effect of the clearness on the response of the system under different exciting conditions. The results indicate that the clearances can hold back the transfer of the vibration and it is rational to prekeep the clearances in the system.