The embodiment of the invention discloses a prime lens. The prime lens comprises first lenses with negative focal power, which are sequentially arranged from an object side to an image side along an optical axis, a second lens with negative focal power, a third lens with positive focal power, a fourth lens with negative focal power, a diaphragm, a fifth lens with negative focal power, a sixth lenswith positive focal power, a seventh lens with positive focal power and an eighth lens with positive focal power or negative focal power. The first lens, the third lens, the fifth lens and the sixthlens are all spherical lenses, and the fifth lens and the sixth lens form a cemented lens. The second lens, the fourth lens, the seventh lens and the eighth lens are aspheric lenses. The prime lens provided by the embodiment of the invention has the characteristic of ultra-large light flux, is particularly suitable for monitoring requirements under complex conditions, can achieve a large apertureof F0.8-1.2, supports an image plane of 1/1.8 inch, and can achieve a field angle range of 90-130 degrees.