The invention relates to a planar rotary worktable of a headstock device. The planar rotary worktable is characterized by comprising a longitudinal slide bedplate, a spindle head angle plate, a locking screw, a rotary positioning shaft assembly and a planar rotary driving mechanism; a spindle head is fixedly mounted on the spindle head angle plate, the spindle head angle plate is arranged on the longitudinal slide bedplate and is connected with the right end of the longitudinal slide bedplate by the rotary positioning shaft assembly, the planar rotary driving mechanism which can drive the spindle head angle plate to rotate around the rotary positioning shaft assembly is arranged at the left end of the spindle head angle plate and the left end of the longitudinal slide bedplate, a long arc-shaped through groove is formed in the spindle head mangle plate, and the locking screw can penetrate the long arc-shaped through groove from top to bottom and is connected onto the longitudinal slide bedplate after penetrating the long arc-shaped through groove. The planar rotary worktable has the advantages that the planar rotary worktable is smart and reasonable in structural design, the headstock device which is supported and mounted by the aid of the planar rotary worktable is wide in workpiece inner circle machinable angle range, and accordingly the machining precision and the machining efficiency can be greatly improved.