Aspects of the invention disclose a variable-magnification monocular, comprising an objective lens group, a prism group and an eyepiece group arranged in sequence along an optical axis direction, wherein the eyepiece group comprises an eyepiece compensation group, an eyepiece variable-magnification group and an eyepiece fixing group arranged in sequence; the focal lengths of all lens groups satisfy the following conditional formula: 1.4<F.sub.31/F.sub.32<0.9 and 2<F.sub.33/F.sub.32<1.7. The eyepiece magnification group and the eyepiece compensation group may move closer to or farther from each other along the optical axis to achieve continuous change of magnification between 60-20. The monocular may achieve continuous adjustment of the magnification of the monocular by moving the eyepiece zoom group and the eyepiece compensation group closer to or farther from each other along the optical axis, which can meet the needs of different observation objects.

An optical system includes, in order from an object side to an image side, a first lens unit having a negative refractive power, a first reflective member, a second lens unit including an aperture diaphragm and having a positive refractive power, a second reflective member, and a third lens unit having a positive refractive power. Following inequalities are satisfied:
1.27<(f1/f)/Npr<0.70
0.60<f2/f3<1.60
where f represents a focal length of the optical system, f1 represents a focal length of the first lens unit, f2 represents a focal length of the second lens unit, f3 represents a focal length of the third lens unit, and NPr represents a refractive index of the first reflective member.