A gear adjusting mechanism of a seat headrest. An inclined sliding block mechanism (5) is provided on a headrest main body (1); one end of the inclined sliding block mechanism (5) is fixed on the headrest main body (1), and the other end of the inclined sliding block mechanism (5) is a free end; and a bayonet (6) is provided on the inclined sliding block mechanism (5). The gear adjusting mechanism of a seat headrest comprises a spring (3), and a headrest handle (2) that can be embedded within the headrest main body (1); the headrest handle (2) comprises a gripping portion and a pull rod (8) that are connected to one another; a tail end of the pull rod (8) is provided with a clamping portion (9); a bottom portion of the clamping portion (9) is provided with a first hook (11), one end of the spring (3) being connected to the first hook (11) and the other end being connected to a second hook (13) on the headrest main body (1); and the clamping portion (9) is clampingly connected to the free end of the inclined sliding block mechanism (5) and the bayonet is driven to move or be locked on a seat main body along a trajectory of the seat main body (4) by means of the push and pull of the gripping portion. The present invention also relates to an adjustable seat. The gear adjusting mechanism of a seat headrest has a simple structure, is convenient to adjust, is particularly suitable for use by babies, and is safer and more reliable.