An optical system for a laser level is described. The laser level includes at least two optical systems which are parallel to and oppositely adjoin to each other in a same plane. Each optical system includes a laser light source, a regular lens, a diffusion lens, a beam-splitting element, and an outer casing as well as the regular lens and the diffusion lens are located between the laser light source and the beam-splitting element. For the optical system of the laser level, the laser beam is sequentially regularized, diffused and split into a fan-shaped light beam with an angle greater than 180 degrees to ensure that the light beams from the optical systems are spread in the same plane. Thus, the laser level generates a disk-shaped light beam with 360 degrees to indicate a circular mark in the same plane.