To obtain high quality InSb crystals, a precision diameter-constant control technology is required in the growth process of large size InSb crystals. The principle and method for controlling the diameter of a crystal during its growth by using the Czochralski Method are presented. The factors, such as temperature and time delay, which have influence on diameter-constant control are analyzed. The precision diameter-constant control is implemented by using a computer aided method in the growth process of large size InSb crystals. The resulting InSb crystal with a diameter of 3in shows its smooth surface from the shoulder. Its dislocation density is less than 10/cm^2.