Fast active Power output Reduction (FPR) of Wind Turbine Generators (WTGs) is an effective way to minimize risks of stability and security accidents of a connected power grid under abnormal conditions such as transmission congestion, DC blocking and short-circuit faults. An accurate FPR can be readily achieved by WTG power electronic drives. However, because of slowly-responsive pitch control, WTG mechanical input cannot be adjusted in a timely manner to match the reduced electrical output. The accumulated surplus energy may lead to rotor over-speed, posing the threat of damage to the WTG mechanical system. Therefore, this paper proposes an FPR scheme of a doubly-fed induction generator-based WTG in effort to avoid rotor over-speed by DC chopper and pitch control coordination. After the FPR command is executed by the rotor-side converter control, the rotor over-speed is first restrained earlier by triggering the DC chopper to dissipate the excessive energy. The pitch angle is controlled at maximal rate to eliminate excessive power, switch off the DC chopper and maintain the rotor speed at the original level over a longer period. The various advantages of the proposed FPR scheme over two typical ones are verified through comparative simulation studies, under scenarios of higher-level power command execution and low voltage ride-through. This work is supported by the Science and Technology Project of State Grid Corporation of China (No. SGSXDK00DJJS1900108).