Abstract: The implementation technology of Real-time multi-tasking operating system RTDOS based on MS DOS 3.3 is discussed. First, some basic characteristics of a real time operating system are introduced; some function in a general real time system are analysed. Then the implementaion of RTDOS is introduced in detail. In addition, the control of the reentry to DOS kernel, the function of task managing, the mechanism for task communication, the real time clock and interruption management supervision are discussed.