The deploying cantilever beam is a model of the typical time-varying structural system. The classical dynamic theory cannot be used to solve the vibration equations due to the changing of mass and stiffness. The vibrations and energy variations are studied based on the governing equations. The analytical expressions of the varying energy are obtained to show the characteristics of the vibration in the stable and unstable region of the system. Numerical method is employed to the vibration analysis and the variation diagrams of energy distributing among different modes in unstable state are discussed. The results show that the rule of energy distributing among the modes is quite different in the stable and unstable states.