With the continuous development of computer numerical control (CNC) system, there are higher requirements of accuracy and speed for CNC system, which brings grave challenge for the current most widely used single-core processor platform. In this paper, based on isomorphic quad core ARM processor platform which is under the Linux system environment with the real-time patch, according to the characteristic of hard real-time tasks, soft real-time tasks and non real-time tasks running parallel in the CNC system, the CNC system tasks are distributed to multi-core ARM reasonably. The improved RM algorithm is used to schedule. Finally execute the real-time performance test for the new scheduling algorithm. This scheduling model can solve the problems of mixed tasks scheduling of CNC system admirably, can well meet the real-time requirement of real-time control of CNC system.