由于界面互扩散的存在，实际的超薄多层膜很难具有清晰的界面结构．假设超薄多层膜为具有周期性梯度折射率的多层膜结构，用直线模型和余弦模型模拟了周期性梯度折射率多层膜的软X射线反射率．结果证明，折射率余弦渐变的多层膜虽然不具有清晰的界面，但它同样具有很高的反射率．

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.