The invention discloses a film thickness detector. The film thickness detector comprises a laser light source, an optical lens group and an image receiver. The optical lens group is used for receivinglaser emitted by the laser light source to generate a first light beam and a second light beam, and the first light beam interferes with the second light beam, wherein the second light beam transmitsa to-be-detected film at least two times. The image receiver is used for receiving interference light, determining interference information and determining the thickness of the film according to theinterference information. Compared with the prior art, the film thickness detector has the advantages that the thickness of the to-be-detected film is determined according to the laser interference information based on multiple laser transmission of the film, tiny refractive indexes and distance changes can be sensed through optical path changes of light rays, and accordingly, the sensitiveness for determining the thickness of the film through the optical path changes is improved greatly.