Abstract: Considering a kind of flexible structure, which has uniform flexible beam,attached to the center rigid body and there are point mass and point moment of inertia.The solution of constrained and unconstrained mode for the equations of motion is analyzed and the general equations of frequency are obtained.Based on these analyses, quantitative comparison between constrained and unconstrained mode frequency with different flexible/rigid body inertia is made.The theoretical basis for modeling and design of controller of such flexible system is provided.