Abstract: In recirculating aquaculture system (RAS), the main solid wastes are uneaten feed, faeces and bacterial flocs. Solid wastes can decrease water quality and aquacultural benefit if without removed effectively. A higher removal efficiency of solid wastes is the guarantee for improving the environmental sustainability of RAS, and it will promote the development and utilization of RAS in China. The larger particles can be removed by settling basin, drum filter, swirl separator, and so on. But it’s difficult to remove fine particles accounting for 95% of total solid wastes smaller than 20 μm and the heterotrophic bacteria can use particulate organic matter for mineralization process. This process not only consumes oxygen, but also produces ammonia, which may aggravate the burden of biofilter for autotrophic nitrifying bacteria to remove ammonia and nitrite. For removing fine particles efficiently, Venturi type of foam separator was optimized by the testing of hydraulic retention time and the air inflow, and combined with relevant theory research, the removal situation of particles with different size ranges was analyzed. In this experiment, the water as original sample from the outlet of drum filter with microscreen mesh size of 125 μm was introduced for batch foam fractionation. The mass concentration of particles in the original sample was (30.33±4.54) mg/L, and the size of more than 98% of them was smaller than 10 μm. The removal efficiency of fine particles was applied as the index for optimization of hydraulic retention time and the air inflow. According to the particle size distribution of the original sample, all particles were divided to 4 size ranges, i.e. ≤10, >10-50, >50-90 and >90 μm, and their concentrations in original sample were (6.56±0.98), (8.68±1.30), (7.75±1.16) and (7.35±1.10) mg/L respectively. And the changing rate of fine particles’ concentration for each interval was applied as the index for analyzing the removal situation of particles with different size ranges. The results showed that the hydraulic retention time and air inflow had significant influences on the removal efficiency of fine particles. When the hydraulic retention time was 2.0 min and the air inflow was 1.3 L/min, the removal efficiency of fine particles was the highest, which was 34.06%±4.37%, and particles size range of the foam wastewater discharged from the foam separator was 1-300 μm, more than 95% of which was smaller than 10 μm. Foam fractionation had a good effect on removing the particles whose size was ≤90 μm, and the removal efficiency of the particles with the size of ≤10 and >50-90 μm was relatively higher than that of >10-50 μm, while it was difficult to remove the particles with the size of >90 μm. Removal efficiency can be increased to some extent with the decreasing of bubbles size and the increasing of hydraulic retention time, but there exists an extreme value, and it suggests that the design and operation of foam fractionation is determined by the nature of the particles and size distribution in the aquaculture water. For the sake of improving the fine particles removal efficiency of foam fractionation in RAS, the fractionation mechanism should be further studied, especially the properties of bubbles and particles, such as how to select the bubble size according to the properties and size distribution of particles in different operating conditions of RAS, and how to apply ozone, a kind of strong oxidizer that has been proved to be efficient for improving the removal of solids in actual RAS.