A headlight optical system and a lamp using the same are provided. The headlight optical system is implemented with the refractive optical system of a single lens. The headlight optical system includes a first incident surface, a second incident surface, and an exit surface. The first incident surface is a horizontal plane. The second incident surface surrounds the first incident surface. The inner edge of the second incident surface is connected with the outer edge of the first incident surface. The second incident surface has an inclined angle with respect to the first incident surface. After the light emitted from the polycrystalline light source is incident on the first incident surface or the second incident surface, the refractive optical system refracts and emits the light from the exit surface.