A method for monitoring the spindle preload amount of a spindle by: S1 obtaining a spindle preload amount through a PPC Preload Analyzer; and S2 obtaining an axial force sensor output of the spindle through an axial force sensor, wherein the axial force sensor output is calibrated using the spindle preload amount that is obtained through the PPC Preload Analyzer; establishing a relationship between the spindle preload amount and the axial force sensor output, then regarding the axial force sensor output as the spindle preload amount, and then monitoring the spindle preload amount by monitoring the axial force sensor output.