A rotary spindle head with gear reducer is provided, which comprises a rotary spindle unit, a fork-shaped type spindle base and a driving unit. The fork-shaped spindle base is for linking up the rotary spindle unit with a machine tool, and the driving unit comprises a torque motor and a gear reducer which are sited coaxially on the fork-shaped spindle base. The gear reducer is connected to the rotary spindle unit to drive and rotate the rotary spindle unit in the fork-shaped spindle base. The gear reducer is designed to lower decelerating rate. Besides, the input and output of the gear reducer are coaxial design, the torque of torque motor could be magnified to several times and maintain higher rotational speed to do the high speed light cutting and high torque heavy cutting work.