A power grid frequency flexible operation system is provided. The system comprises a generating unit, which includes a base-load unit and a peak-load unit; a high voltage direct-current (HVDC) transmission unit, which transmits the power generated in the generating unit as direct current (DC) power; and a load, which is supplied with the power generated by the generating unit; wherein the high-voltage direct current (HVDC) transmission unit comprises a converter, which transforms to direct current (DC) power, alternating current (AC) power generated in the generating unit and having a first frequency variation allowance range; an inverter, which is connected to the converter and transforms the direct current (DC) power to alternating current (AC) power having a second frequency variation allowance range, wherein the first frequency variation allowance range is larger than the second frequency variation allowance range.