The present disclosure relates to the field of imaging technologies and, particularly, relates to a varifocal optical system. The varifocal optical system includes a front fixing group, a triple prism, a zooming group, a compensating group, an objective lens group and an image plane which are successively arranged along an incident direction of light, and a total focal length of the varifocal optical system is a fixed value; the front fixing group includes a first lens; the zooming group includes a second lens; the compensating group includes a third lens and a fourth lens arranged along an optical path direction; the object lens group includes a fifth lens; and the first lens, the second lens, the third lens, the fourth lens and the fifth lens are all glass lenses. The present disclosure can satisfy the developing trend of miniaturization on electronic devices, and has a better imaging performance.