Traditional climbing search algorithm cannot usually focusing accurately due to the interference of evaluation function local extremum effected by measure environment. A searching algorithm of auto-focusing based on MTF auxiliary focal judgment was introduced to improve the climbing method. Definition evaluation function and MTF values of images were considered at the same time. And based on those results, search direction was judged. Slanted-edge method was improved to calculate the image MTF values accurately based on the auto-selection of slanted-edge area. At last, the principle and implementation of the improved algorithm was given. In the theodolite auto-fousing system, the system imaging effect was validated, and different initial position and circumstance were considered using the proposed searching algorithm. The experiment results of theodolite auto-focus system show that the improved climbing search algorithm can effectively eliminate the local extremum disturbance, and make the system searching focusing accurately and reliably.