In order to study the structure and performance indexes of main materials of combined filter of drainage holes for dam foundation in Xiangjiaba project,we use combined filter to protect the drain holes which are drilled through the weak soil layers.On the basis of the results of permeability experiments of weak soils in dam foundation and indoor experimental results of the main materials such as geotextiles and foamed plastic,the structure of combined filter and main performance indexes of the materials are determined in association with engineering experiences.The filtering and silting experiments show that the combined filter is effective to protect the weak soil layers,with little silting in long-term operation.