An eyepiece optical system and a head-mounted display are disclosed. The optical system comprises a first lens, a second lens, a third lens, a fourth lens, and a fifth lens arranged coaxially and successively along an optical axis direction from an eye viewing side to a displayer side, wherein a focal length of the second lens is f2, a focal length of a lens group formed by the third lens and the fourth lens is f34, a focal length of the fifth lens is f5, a distance from a display of the optical system to the fifth lens proximate to a surface of the display is fd, and a total system length is fw. When a particular relation is satisfied, on the basis of cost reduction and weight reduction, significant elimination of system aberration as well as high-quality optical indices can be achieved.