Abstract: The fluidity of powder has an important impact on the key production processes such as mixing, conveying and packaging. To solve the fluidity problem in the production of purified frocto-oligosaccharide (P-FOS) powder, the influence of different particle sizes and their proportions on the fluidity of P-FOS powder was investigated. The flow characteristic of P-FOS was analyzed by using the powder rheometer. Research on the mixing experiment based on the different particle sizes (180~250, 150~180, 120~150, and <120 μm). Then, the regression model was carried out using particle size as an independent variable and the powder flow index ff as an dependent variable. The results showed that there was no significant difference in the content of frocto-oligosaccharide with different particle sizes (P>0.05), but the fluidity of P-FOS dry powder with the reduction of particle size. P-FOS dry powders with different particle sizes exhibit fluidity across different regions. The flow indices of samples with particle sizes in the range of 180~250, 150~180 and 120~150 μm were 12.5, 7.7, and 6.6, respectively. Their flow features were in the zone of easy flow area. Meanwhile, the flow indices of samples with particle sizes of 109~120 and 96~109 μm were 2.9 and 3.3, respectively. Their flow characteristics were in the bonding region. The mixing ratio of particles of different particle sizes had a significant influence on fluidity. The relationship between the particle size and the flow index was consistent with the square regression model, and the R² (adjusted) could reach to 99.43%. If the particle size distribution was known, the fluidity of the P-FOS powder could be predicted and regulated by this model, thus guaranteeing the continuity and stability of the transport and packing of the dry powder.