The steady-state response of forced power oscillation is presented in multi-machine power systems with complex modes superposition method. The resonance mechanism and the key factors that influence the oscillation amplitude are analyzed. The similarities and differences between the steady-state resonance response of forced power oscillation and the response of traditional negative damping low frequency oscillation are discussed in multi-machine weak damping systems. Some basic properties of forced power oscillation in multi-machine power systems are set forth. It contributes to a better understanding of forced power oscillation and oscillation characteristics. Finally， a series of simulation analysis is carried out to indicate the feasibility and validity of the analysis.