Auto-focusing is an important step in development a computer-aided evaluation system for diagnostic microscope. An auto-focusing algorithm can identify the best focus reliably and objectively, and prevents the tedious and time consuming steps of manual focusing. In this paper, a coarse-to-fine auto-focusing method is proposed based on a multi-resolution scheme which locates the focusing range by quadratic polynomial fitting in low resolution and achieves accurate focal point by using binary search in fine resolution. An algorithm was developed to identify the focal point under multiple image resolution with the Sum of Modified Laplacian (SML) as the focusing value. The main contribution of this system is to reduce the auto-focusing time. Comparing with the conventional binary search, the coarse-to-fine method only needs half of the time to accomplish auto-focusing. Using the proposed technique, the efficiency of auto-focusing system has been significantly improved.