Aiming at the Computer Numerical Control (CNC) machining system of micro lathe, a convenient open CNC system based on USB bus is proposed and implemented initially. The open CNC system was consisted of a lower computer consisting of STM32+Field Programmable Gate Array(FPGA) and a host computer. The software implementation of real-time multitasking scheduling, decoding, tool compensation and interpolation algorithm for numerical control (NC) system was studied. The functional requirements of the lower computer were studied for the micro lathe and the hardware circuit board was designed. The memory management and motion control of the lower computer were studied. The communication method based on Universal Serial Bus (USB) was proposed. The feasibility of USB in open CNC system was verified. The experiment proves that the USB bus can be conveniently and efficiently used in 〖JP3〗the open CNC system; the combination of software and hardware ensures that the system has good openness, real-time and synchronization.〖JP〗