A projection optical system: a first lens group (G1) is used for focusing; a second lens group (G2), a third lens group (G3), and a fourth lens group (G4) are used for zooming; a diaphragm (ST) is located between the third lens group (G3) and the fourth lens group (G4); at least one surface of a first lens (L1) comprised in the first lens group (G1) is aspherical, which helps to improve the field of view of the optical system and better correct off-axis aberrations and distortions; and at least one surface of a fifth lens (L5) comprised in the fourth lens group (G4) is aspherical, and an aspherical lens is arranged behind the diaphragm (ST), which may well correct optical system aberrations to improve MTF performance. By means of optimizing the design of the number of lenses of each lens group, the surface shape of each lens, and optical parameters of the lenses, the present projection optical system is able to meet the requirements of a large aperture, a simple and miniaturized structure and low costs, and may effectively ameliorate system aberrations and inhibit the occurrence of system distortions.