The removal of suspended fine particles (<30 μm) in high density recirculating aquaculture system is the key to water environment regulation. In this paper, the Coulter particle size analyzer was used to analyze the temporal and spatial distribution of suspended fine particles in the recirculating aquaculture system for Salmo salar. The results showed that: in the system, the particles with particle size of 1~3μm, 3~15μm and 15~30μm accounted for 98.36%, 1.63% and 0.01%, respectively. The number, total volume and total surface area of suspended fine particles in the system were (0.54~1.5)×106 /mL, (0.97~6.3)×106 mm3/mL and (0.24~1.3)×107 mm2/mL, respectively; for the particle number distribution, with the decrease of particle size, the number of particles increased rapidly; for the volume distribution, the larger the particle size, the larger the total volume of particles; for the surface area distribution, with the increase of particle size, the total surface area decreased first and then increased; foam separator and submerged biofilter showed strong retention ability of suspended fine particles. The purpose of this study is to reveal the distribution of suspended fine particles in a stable circulating aquaculture system, and provide theoretical guidance and reference for selecting different solid-liquid separation technologies and improving the removal efficiency of solid particles.