According to similarity theory, the paper chose a large space building and established an experimental table of liquid water model with 1∶15 reduced-scale to study flow field characteristics around the return air grille in a large space building air-conditioned with low-side wall air supply and medium-height air return.It took salt water as fluid medium and chose the polystyrene particles whose diameter was from 10 to 100μm as tracer particles to calculate their flowing behaviors and accuracy in tracing non-uniform flow field around the air return grille. Three indexes such as velocity amplitude ratio η, settling velocity Ug and delay velocity Us were used to judge whether their flowing characteristics were good.The results indicate that when the density ratio σ between the particles and the fluid is less than 1.0, the smaller the diameter of particles, the smaller the velocity amplitude ratio η is, and η is close to 1.0, which means that the flowing behaviors is better. While the density ratio σ is larger than 1, the smaller the diameter of particles, the larger is its velocity amplitude ratio η, and the closer the value of η is 1,the better are the flowing behaviors. The better flowing behaviors represented that the tested flow field could truly reflex the actual flow field Characteristics. The paper used PIV technique to get the velocity information of air return flow field. The results indicated that under certain experiment conditions, the settling velocity Ug has no influence on particle movement and the delay velocity Us is very little, which demonstated the flowing behaviors of the particles are fairly well in the salt water fluid medium.