In order to further explore the effects of particle size and substitution rate of waste rubber particles on the mechanical properties of cement mortar under static loading, three kinds of rubber particle sizes (0.100, 0.425 and 2.000 mm) were selected to replace natural sand with volume fraction of 0%, 5%, 10%, 15% and 20%, respectively. The compressive, flexural and splitting tensile strength tests were carried out, and the influence mechanism of rubber particle on the mechanical properties of cement mortar were studied by mercury intrusion porosimetry test, scanning electron microscopy analysis and energy dispersive spectroscopy analysis. The results show that the compressive, flexural and splitting tensile strength of rubber cement mortar decrease with the increase of rubber substitution rate, and the effect of particle size on the strength of mortar is not obvious. When the rubber substitution rate increases from 5% to 20%, the 28 d compressive strength of rubber cement mortar decreases by 27.2%~61.0%, the flexural strength decreases by 22.5%~31.7%, and the splitting tensile strength decreases by 30.0%~38.9%, respectively. The ratio of bending-compression ratio increases with the increase of substitution rate and particle size. The total porosity and average pore size of rubber mortar specimens are negatively correlated with compressive, flexural and splitting tensile strength.