Data exchange between NC and PLC was carried out by automatically identifying the type of function accessory by detecting switch signal. The subroutine was selected and executed by using FB4 and FC9 of the Siemens standard program function block. The automatic compensation for tool measuring size of function accessory of the NC machine tool was realized finally by receiving the external zero offset value, which solves the problems of multiple function accessories and repeated tool measuring with multiple tools of one machine tool, low processing efficiency, and so on.