The power system forced power oscillation will be excited by the continuous cyclical small power disturbances. Its steady-state response during resonance is very similar to the response of the traditional weak damping low frequency oscillation. The energy conversion characteristics of the forced power oscillation in a linear system during the steady-state resonance is analyzed firstly using energy functions with the linear equations of motion of the rotor. Using this energy conversion theory， the difference between these two kinds of oscillations is obtained. Then， basic principles to identify the disturbance source of the forced power oscillation by energy function of linear system are proposed with a two-machine system. Finally， based on the structure preserving model of multi-machine power systems， the energy function is extended to the key branches and buses， which is useful to online identify the direction or location of the disturbance source of the forced power oscillation by simply using the network dynamic information. The simulation results of both the 4-machine 2-area system and the New England 10-machine 39-bus system indicate the feasibility and validity of the proposed method.