The invention relates to a long-optical-path optical system which comprises an ultraviolet light source and an infrared light source. The optical system further comprises the components of a collimating unit which comprises a first lens and a second lens, wherein a first dichroic mirror is arranged between the first lens and the second lens, after light emitted from the infrared light source is collimated by the first lens and is transmitted through the first dichroic mirror, the light is collimated by the second lens and is further transmitted, and light emitted from the ultraviolet light source is reflected by the first dichroic mirror and is collimated by the second lens for being transmitted; a focusing unit which comprises a third lens and a fourth lens, wherein a second dichroic mirror is arranged between the third lens and the fourth lens, transmitted infrared light is focused by the third lens and then is transmitted through the second dichroic mirror, afterwards is focused by the fourth lens for being detected, and ultraviolet light is reflected by the second dichroic mirror and is focused by the third lens for being detected; and a detecting unit which comprises a first detector and a second detector, wherein the first detector and the second detector respectively detect the infrared light and the ultraviolet light. The long-optical-path optical system has advantages of simple structure, high collimating effect, etc.