The invention discloses a subsynchronous oscillation suppression method for a wind power plant flexible DC grid-connected system based on an impedance method. The method comprises the steps of establishing an equivalent impedance model of a doubly-fed wind power plant and an equivalent impedance model of a VSC-HVDC rectifier station; calculating the impedance ratio of the VSC-HVDC rectifier station to the doubly-fed wind power plant to obtain an amplitude-frequency characteristic of the impedance ratio of the interconnected system; obtaining lower limits of a proportionality coefficient and anintegral time constant according to the relation that a resonance peak value of the amplitude-frequency characteristic of the impedance ratio of the interconnected system changes along with parameters of an alternating-current voltage controller, obtaining upper limits of the proportionality coefficient and the integral time constant according to the relation that the bandwidth of an alternating-current voltage ring of the VSC-HVDC rectifier station changes along with the parameters of the controller, calculating to obtain an optimized value range of control parameters of the alternating-current voltage controller, changing the control parameters, and inhibiting subsynchronous oscillation. According to the invention, the stability of the wind power plant flexible DC grid-connected systemis improved, and the difficulty of engineering implementation is reduced.