Effect of the variation of sand grain size on cavitation erosion of concrete in sand-bearing water flow was experimentally investigated via a small looped water tunnel. Sand grains with five typical median particle sizes (0.08, 0.25, 1, 2 and 3 mm) were obtained by sieving method. For each grain size, the sand-bearing water samples with the same sand content S=12 kg/m³ for each given sand grain size were prepared respectively. The sand content of high-speed sand carrying water flow was real-timely measured by an infrared suspended solid analyzer. The pressure at the working section of the water tunnel for each sand-carrying flow with a given grain size was acquired by pressure data acquisition system in real-time, then of which a comparison was made with the case of clear water. Concrete specimens with water-cement ratio W/C=0.4, cement-sand ratio C/S=1.5，7 d aging strength f_cu,k=17.8 MPa were poured. Afterwards, the specimen was placed into a specimen box fixed in the working section of water tunnel to undergo cavitation erosion test for 4 h by different flow speeds for one sand carrying water with a given sand grain size. The results showed that for the case with send content of S=12 kg/m³ and flow speed V=38.2 m/s, the flow pressure at cavitation zone decreased with the increase in sand grain size, which resulted in decrease in the incipient cavitation number and promoted the formation of cavitation; the sand carrying flow pressure at cavitation-erosion zone increased with the increase in sand grain size, which could promote the cavitation erosion. The cavitation erosion level of the concrete specimen was aggravated with the increase in sand grain size, and the cavitation erosion range was further spread with the increasing flow velocity.