Abstract: A series of triaxial tests have been conducted to study the effect of coarse grain percentage and size, dry density of based soil and confining pressure on the mechanical properties of "based-type" granular materials. The rule and mechanism of the effect of size and contents of coarse grains on the strength and deformation characteristics of field materials are mainly discussed according to the test results. It is put forward that this effect can be reflected comprehensively by the index-the number ofcoarse grains per unit length on the shear line. Bifurcation of deformation state of test specimens caused by coarse grains is analysed theoretically.