The invention provides a long-focus ultra-high-definition day-night confocal optical system, and belongs to the technical field of an optical system. The long-focus ultra-high-definition day-night confocal optical system comprises a first lens, a second lens, a third lens, a fourth lens, a fifth lens, a sixth lens and a light sensing chip which are sequentially arranged from an object surface to an image surface along an optical axis, and is characterized in that a diaphragm is arranged between the second lens and the third lens, the first lens, the second lens, the fourth lens, the fifth lens and the sixth lens are glass spherical lenses, the third lens is a plastic aspherical lens, and focal lengths of the first lens, the second lens, the third lens, the fourth lens, the fifth lens and the sixth lens are sequentially negative, positive, negative, positive, positive and negative. The optical system provided by the invention not only can overcome the defects such that the number of lenses in the optical system is great and the resolution under a long focus is unsatisfactory, but also can reduce the size of the lens and reduce the production cost at the same time.