A projector including an optical machine, a lens and a casing is provided. The optical machine is configured to form an optical image. The lens is located on a projection path of the optical machine so that the optical image is projected on the lens. The casing is configured to receive the optical machine and the lens. The casing is provided with a positioning mark, which is aligned with an imaging center of the optical image in a vertical direction and configured to align the imaging center of the optical image with a center of a projection screen.