Abstract: A processing device with a constant force spindle capable of quickly switching a force control axial direction and a processing method thereof, having a quick-detach unit fixed at a constant force control unit by a first quick-connecting piece, and respectively connecting and moving synchronously with the spindle by a second quick-connecting piece and a third quick-connecting piece, the first quick-connecting piece moves along with a working end of a multi-axial transfer unit, and when combined, a first axial direction that is coaxial with the force control axial direction is defined; alternatively, the first quick-connecting piece moves along with the working end of the multi-axial transfer unit, and when combined, a second axial direction that is coaxial with the force control axial direction is defined, the first axial direction is not parallel to the second axial direction and has an included angle.