Systematic ring shear tests of the sand clay in an ancient landslide were carried out in laboratory to revealthe effect of coarse grain content on the residual shear character and residual strength of slip zone soils,and a backanalysis was carried out with multiple similar sections.Then,the calculated residual strength was comprehensivelycompared with that obtained by laboratory test,with a suitable residual strength of ancient landslide being recom-mended.The results show that (1) the ring shear test is an ideal method to obtain the residual strength under largeshear displacement; (2) different from common clay,the slip zone soil is of no obvious strain- softening character,and the strain softening becomes weaker when the coarse grain content increases; and (3) at high coarse grain con-tent,the equivalent residual internal friction angle nonlinearly increases with the coarse grain content.