In order to exert the full potential of 4-axis motion controlling development platform, an experiment semi-physical simulation system of 3-axis NC milling machine is developed by taking it as a research object. This system was combined of software functions with hardware control by function library, which realized hardware control method by software. The path simulation and machining simulation functions were realized based on OpenGL library and timer built-in. This system could be run not only on a single computer for path simulation, but also online to control running of motor and simulate practical paths, in order to adapt to different experiment fields. By tests, it is proved that this system can fully exert the latency potential function of test devices, which is helpful to train the design ability of students for NC system.