The invention discloses a sighting telescope optical system for a gun, comprising an objective lens group, a transferring lens group and an eye lens group which are sequentially arranged from an object side to an image side. The transferring lens group is composed of a first positive lens group, a second positive lens group, a third positive lens group and a fourth positive lens group, wherein, the first positive lens group is arranged fixedly, the second positive lens group and the third positive lens group are arranged movably and the fourth positive lens group is arranged fixedly; and the first positive lens group, the second positive lens group, the third positive lens group and the fourth positive lens group are arranged sequentially from the object side to the image side. The sighting telescope optical system has simple structural design and processing, can obtain larger zoom ratio and relatively larger sighting telescope field of view, and reduce the field angle of the outgoingbeam of a transferring system, thus correspondingly decreasing the lens aperture of an eye lens system at the rear end of a zoom system.