The common cushion layer used in the geomembrane faced rockfill dam is usually made of granular material such as gravel or cobble. Under the action of water pressure， the puncture of geomembrane is likely to occur at the protruding part of the cushion surface，and the bursting of geomembrane may occur at the void. The rougher surface of the cushion results in the greater possibility of bursting and puncture of the geomembrane under the same conditions. However，there is not a suitable quantitative index to describe the roughness of the surface of the particle cushion. In order to solve this problem， this paper starts from the puncture behavior of geomembranethis and uses progressive method to analyse the surface line of geomembrane particle cushion and a new quantitative index was proposed based on tthe distribution of convex shape. The index is then compared with JRC indexes，and it indicated that the quantitative index proposed in this paper can directly reflect the possibility of geomembrane puncture or bursting occurrence，and JRC indexs is not suitiable to analyse the roughness of granular cushion surface.