The invention discloses a frequency domain impedance-based secondary synchronous oscillation risk online evaluation method and device; the method is characterized in that a potential secondary synchronous oscillation mode and a key plant station are screened on the basis of the amplitude-frequency characteristic quantity of an actually-measured inter-harmonic signal; on the basis of a power grid real-time operation mode, each branch secondary synchronous admittance corresponding to the frequency of the potential secondary synchronous oscillation mode is generated on the basis of equipment frequency domain impedance modeling; and based on a secondary synchronous admittance matrix, a secondary synchronous oscillation key plant station serves as a driving point, and driving point source sideand network side polymerization equivalent impedance values are calculated, and further the resonance and damping margin of the potential secondary synchronous oscillation mode based on the driving point polymerization equivalent impedance values are calculated. The method can meet the requirements for quantitative evaluation of secondary synchronous oscillation risk and online safety pre-warningin the current operation mode of the power grid.