In this article we discuss the design of double-crystal monochromator control system of 3B3 middle-energy beam line in Beijing Synchrotron Radiation Factory. The multimedia timer and multithread technique were used to solve the high speed data transmission in serial port system and data communication in multi-serial port system. The real time graph drawing and dynamic zooming in double-crystal monochromator debugging system accomplished by VC++6.0 were also described.