The utility model discloses a dynamic balance screw cap; a shaft hole is formed inside the screw cap; an embedding part with a smaller diameter is formed at the bottom end of the shaft hole; an annular locking ring is formed at the top side of the embedding part; the locking ring is eccentrically arranged relative to the axis of the shaft hole; a plurality of gripping parts are formed on the external circumferential surface of the screw cap at equal intervals; a counterbalance sunken hole is concavely arranged at one side of the external circumferential surface of the screw cap, and the side is protruded inward relative to the locking ring; through the counterbalance sunken hole, the utility model reduces the weight at one heavier side of the screw cap; therefore, when the screw cap is spirally locked to a cutter holder, the screw cap can make high-speed rotation at a stable location of center of gravity to reduce vibration, extend service life and achieve high processing accuracy.