A fixed-focus lens includes a first, a second, and a third lens groups arranged in sequence from the enlarged side to the reduced side and having positive refractive powers. The first lens group includes a first, a second, and a third lenses from the enlarged side to the reduced side. The second lens group includes a fourth, a fifth, and a sixth lenses from the enlarged side to the reduced side. The third lens group includes a seventh lens. The lens of the first lens group closest to the enlarged side in the fixed-focus lens has a concave surface. The distance between the lens surface of the second lens group closest to the reduced side and the lens surface of the third lens group closest to the enlarged side is L1. The overall length of the fixed-focus lens is L. The fixed-focus lens satisfies 0.1