The friction characteristic of the interface between composite geomembrane and coarse sand-gravel is of great importance to the structural stability. Most of the present experimental studies focus on the contact between soil of finer particles and geotextile, but research on the characteristics of interface between gravel of coarse soil particles and geomembrane is rarely seen. In this paper, direct shear friction tests were conducted for the interface by using large-scale direct shear apparatus. The influence of gradation and relative density on the friction characteristics are discussed. Results show that in the presence of equal relative density, the friction factor increases with the increase of coarse particle content, and in the presence of equal gradation, the friction factor increases with the increase of relative density. Furthermore, the influence of the fixation pattern between geomembrane and the lower base of shear box are analyzed. Test results reveal that the discrepancy of interface friction angle in different fixation patterns could be as great as 2 times.