The invention belongs to the technical field of ultra-precise processing, and discloses a tool dynamic balance online measuring and regulating device for ultra-precise fly-cutter milling processing. The tool dynamic balance online measuring and regulating device comprises a mechanical measuring and regulating system and a data acquiring and processing system, wherein the mechanical measuring and regulating system comprises a spindle box, an air bearing main shaft, a workbench, a fly-cutter system and two groups of sensor assemblies; the fly-cutter system comprises a fly-cutter disc, a diamondfly-cutter and a diamond fly-cutter counterweight; each group of sensor assemblies separately comprises a mounting base, an adjustable movable platform, an adapter plate and a laser displacement sensor; and the data acquiring and processing system comprises a computer, a serial port communication cable, a sensor controller and a sensor cable. Zero phase and the displacement value of the fly-cuttersystem are separately acquired through two laser displacement sensors, so that dynamic balance regulation is completed in a balance bolt weighting mode, and therefore, dynamic balance measuring and regulating of the fly-cutter system can be quickly and precisely completed.