Based on the STEP-NC basic theory, this paper uses the open module design method to construct a kind of CNC turning system based on STEP-NC in the integration environment of VC++6.0 and ST-Developer. The construction of system and processing of the programme and data are accomplished by using MFC class libraries, STIX and ROSE libraries, etc. This paper also studies the key technologies of the system, such as turning feature recognition, STEP-NC interpreter and tool-path generator. In the end, the STEP-NC turning simulative machining platform which can validate the correctness of STEP-NC program machining is constructed by using OpenGL technology.