The invention discloses a cooperative control method for a wind power plant and a flexible direct current transmission system for improving an inertial level of a power system. The control method is used for simulating the inertia of a synchronous generator by virtue of a direct-current capacitor and the kinetic energy of a DFIG (Doubly Fed Induction Generator) rotor. The cooperative control method comprises the steps that a GSVSC (Grid-Side Voltage Source Converter) enables the direct-current capacitor to correspondingly absorb or release energy through the slip frequency control on a direct-current voltage under the disturbance of a power grid; then, a WFVSC (Wind Field Voltage Source Converter) converts the changed direct-current voltage into a wind field side alternating-current system change frequency signal through frequency conversion control after feeling the fluctuation of the direct-current voltage, so that a maritime alternating-current system and a shore alternating-current system are artificially coupled, and the communication between convertor stations at two ends is omitted; finally, a power instruction value is adjusted by a DFIG power controller in order to respond to the frequency change of the WFVSC, and the rotating speed of the rotor is also correspondingly changed. Through a series of cooperative control, a maritime wind field takes part in the frequency adjustment of a power system so as to play an important significance to the introduction of a large maritime wind field.