Cement mortar detached from the concrete surface is one of the main damaged forms of concrete structures caused by debris flow. To enhance the abrasion resistance of cement mortar by improving its micropore structure and surface properties, the effects of hydrophobic treatments (integral treatment, surface coating), a dosage of hydrophobic agent (2 wt%, 4 wt%), and solid particle size (4.75-7, 7-9.5, 9.5-13.6 and 13.6-25 mm) on abrasion performance of cement mortar are investigated. The results show that the abrasion rate and abrasion depth of the mortar significantly increase with the increase of solid particle size of debris flow. At 120 h, the abrasion rate of the mortar increases by 85-151% under a solid particle size of 13.6-25 mm, compared to that of solid particle size of 4.75-7 mm. An experimental equation is proposed to describe the relationship between abrasion rate and solid particle size. The hydrophobic treatment does not show better abrasion resistance due to reduced mechanical strength. The abrasion mechanism caused by debris flow on cement mortar is mainly the collision and friction generated by the saltating and sliding solid particle. Moreover, the solid particle varies from a large irregular particle to a small smooth particle during the debris flow abrasion process. Debris flow abrasion shows a more significant effect on flexural strength, compared to compressive strength.