In order to realize real-time auto-focusing for theodolite,lens-apart method was designed with the corresponding math model built and analyzed.An Object beam splitted after the first image surface of the theodolite traveled through two dialyte lens and then was imaged on CCD.The distance between two spots when exactly focused was set as the calibration value,and the relationship between defocus distance and spot offset was given.The influence of tracking center shift and divergence angle etc.on focusing accuracy was analyzed.Compared with the traditional focusing method,this scheme has a higher accuracy，faster calculation，higher tolerance for environment temperature and a simpler construction.The experiment result exhibited a resolution higher than 0.056mm,which shows a bright engineering application prospect for this method.