The modal series is a novel method for the representation of the dynamic characteristics of a nonlinear system. In this paper， the modal series method is applied to investigate the dynamic responses of HVDC/AC power systems. A comprehensively analytical technique based on the modal series method and symbolic computer algebra is proposed for the analysis of power systems that include HVDCs. Using this method， the oscillation modes and non-linear dynamic behavior of power systems subjected to large disturbances can be studied. The results are suitable for complex power system stability analysis and control system design. The proposed method is general and may be extended to include other FACTS devices. The nonlinear time-domain simulation results of a test system demonstrate the validity of the proposed method.