A tool-path calculation apparatus for a numerical controlled system and a method for operating the same are applied to a CNC tool machine. The tool-path calculation apparatus includes an upper controller and a servo driver. Firstly, an interpreter is provided to interpret the tool paths to produce a plurality of executable instructions. Afterward, the executable instructions are sent from the upper controller to the servo driver through a serial communication interface and stored in a queue buffer. Finally, the executable instructions are received and calculated by a tool path calculator to produce a plurality of points along the tool paths.