Actual ultra thin multilayers are difficult to have a clear-cut interface due to interface interdiffusion. It is assumed that ultra thin Multilayers have a periodic graded-index multilayer structure.A line model and a cosine model are used to simulate soft X-ray reflectivity of periodic graded-index multilayers. The simulations show that periodic graded-index multilayers with cosine model have a higher reflectivity. it's significative for design of ultra thin Multilayers.