Abstract: The working mechanism of two kinds of over-travel protection for CNC machine tool is introduced. When one certain axis of machine tool triggers alarm of over-travel in both directions, the current PLC status of machine tool can be inquired by means of system diagnostic function. Combining with the electrical measurement of the machine tool circuit, the fault causes can be found and analyzed. By adjusting the position of the travel switch and modifying the relevant system parameters, the over-travel fault of the CNC machine tool is finally eliminated.