Abstract: The mechanical properties of soil under reciprocating shear are of great significance to the study of cyclic load response. By improving the reciprocating shear direct shear instrument, 4 normal stresses are set for 32 cycles of shear. The behavior of stress-strain curve and the law of hysteresis loop area and damping ratio are found out for sandstone-mudstone particle mixture. According to the distribution law of shear stiffness with normal stress and reciprocating shear frequency, an empirical calculation method for shear stiffness of sandstone-mudstone particle mixture is established. By generalizing the contact mode between particles, the micro mechanism of strength and deformation is revealed for sandstone-mudstone particle mixture. Experimental results and theoretical analysis show that the stress-strain curve gradually changes from strain softening to strain hardening as the normal stress increases. The area of the hysteresis loop gradually decreases with the increase of the number of reciprocating shears, and the damping ratio increases slowly with the increase of the number of reciprocating shears, and has nothing to do with the normal stress. The initial shear stiffness is linear with the normal stress, and the normalized shear stiffness has a logarithmic function relationship with the number of reciprocating shears. Based on this, an empirical calculation of the shear stiffness of the sand-shale mixture considering the normal stress and the number of reciprocating shears is established.