Abstract: The tests on the shear strength parameters of the compacted sandstone-mudstone mixtures as well as the analyses of their influencing factors are meaningful to guide the stability evaluation, design and construction of ultra-high fill slopes. Taking a natural gas processing plant in Mianyang City, Sichuan Province as an example, 13 field large-scale shear tests from 4 ultra-high fill slopes are carried out to study the effects of the rock content, water content and normal stress on the shear strength parameters. The results show that: (1) The comprehensive shear strength parameter of sandstone and mudstone mixtures is 34.93 kPa, and the internal friction angle φ is 31.38°. (2) The shear strength parameter c decreases with the increase of the stone content, and the internal friction angle φ increases with the increase of the stone content. (3) The shear strength parameters c and internal friction angle φ of mixed fill decrease with the increase of the water content. (4) The influences of the water content on the shear strength parameters are greater than those of the stone content, and the change of the water content should be paid attention to during construction. The research results are of great significance for ensuring the stability of ultra-high fill mixture slopes.