This paper introduces a method of auto focusing based on fuzzy-control. Using this method, we design the scheme of optimization for focus. Variance function is used as approximately evaluate function for a large-scale in this system, and Square-gradient function is used as precise evaluate function in certain scale. Fuzzy-control is adopted the control of optimization for focus. The experiment testified that auto-focusing in this system is so accurate that accord with the required precision, and the image of collecting is clear. The machine of auto-focusing is simple, and the control of whole system is easy to implement.