Focusing systems play an important role in a variety of optical measurements and detection instruments. In this paper, several kinds of auto-focusing methods were described and the relevant focusing evaluation functions were introduced based on image processing methods. An auto-focusing system based on image processing was designed by using the TMS320DM642 digital signal processing chip as a platform, and its programming was implemented by a real-time multi-tasking operating system DSP/BIOS developed by TI company. The real-time multi-tasking operating system provides a guarantee for the realization of task scheduling and real-time focusing. Moreover, some problems and solutions which are needed to pay attention in the realization of auto-focusing system were demonstrated. The system designed in this paper can find the image plane in 4-5 steps(about 2 s), which meets the requirements of practical applications.