Open CNC system has many characteristics, such as logic independence of module, interface standardization,flexibility and portability.It has become the main development trend of NC system. An embedded software PLC open CNC system was designed based on the TwinCAT system and it was applied to CAK6136 lathe. PC design, selection of the EtherCAT module and debug of the module parameters, Gcode decoding,interpolation,ADS communication and the debug of lathe were conducted.