The invention discloses a cooperative frequency modulation method and system for a fan and synchronous machine under inertia control and based on convertor control. The cooperative frequency modulation method comprises the steps that after disturbance occurs, frequency drop occurs to the system; the synchronous machine measures an initial frequency changing rate; the initial frequency changing rate is used for calculating actual active unbalance amount; a simplified system frequency responding model is used for calculating virtual system frequency offset amount; the virtual frequency offset amount is used for replacing synchronous machine rotation speed changing amount as input signals of a speed adjusting system; and fan actual additional transmitted power is calculated, and fan actual output power is obtained by adding a constraint condition under the premise of the fan actual additional transmitted power. According to the scheme of the cooperative frequency modulation method and system for the fan and synchronous machine under the inertia control and based on the convertor control, the synchronous machine is enabled to additionally transmit power under the condition that systemfull loads are unbalanced, and no communication device needs to be arranged between the synchronous machine and the fan. A provided cooperative frequency modulation strategy can further modify a phenomenon of secondary frequency drop caused by withdrawal of the inertia control.