Abstract: The cap capability of capping layer was essential for the preservation of coalbed methane reservoirs.The size of the cap capability was mainly controlled by four factors:lithology, thickness, depth and tectonic deformation of the capping layer.In this paper, silt ratio,the equivalent coefficient bed thickness, depth and structure of the curvature of capping layer were selected as the four evaluation factors.Based on AHP analytic hierarchy process,through the four evaluation factors quantify graded and given different weights,using the mathematical model of sealing capacity index ,this paper realised quantitative evaluation on the cap capability of capping layer of Gaxun exploration area in Yuqia Coalfield.