Hydroelectric generation system is mainly composed of penstock, hydro-turbine, generator, servicing facility and power load, its stability is directly related to the dynamic characteristics of each subsystem, but not completely dependent on the behavior of the subsystems. To better study the transient energy characteristics and stabilization mechanism of the hydroelectric generating set in the sudden load decreasing transient. And make full use of strengths of generalized Hamiltonian system in describing energy flow, the Hamiltonian model of a hydroelectric generating set including the turbine, water diversion system and generator is established by the method of orthogonal decomposition. Firstly, the energy flow of the hydroelectric generating set in the framework of generalized Hamiltonian theory is proved theoretically to be consistent with the real system, and the transient process of sudden load decreasing can be described effectively. Moreover, the variation laws of the flow, the rotating speed and the power angle of the set in the sudden load decreasing transient are studied respectively. The results indicate that the constructed Hamilton function can effectively describe the energy change of the system. It provides theoretical support for the stable operation of the hydroelectric generating set in the sudden load decreasing transient, and a new research idea for the stable operation of the set in other transient processes.