The effect of different particle sizes and processing techniques on the nutrients and physicochemical properties of millet flour was investigated. The processed samples, polished millet flour (POF) and germinated brown millet flour (GEF), and brown millet flour (BRF) were sieved to obtain 6 particle sizes, then determined the content of reducing sugar, starch, amino acid, protein and the parameters (color, viscosity, gelatinization temperature, crystallinity and microstructure) of millet flour. The results showed that smaller particle size could have more reducing sugar and amino acid, lower viscosity, less redness and yellowness, more lightness and higher gelatinization temperature. Germination increased reducing sugar content but consumed starch. Besides that, germination also improved amino acids and protein content, and changed proportion of amino acids. Gelatinization temperature and thermal stability increased but viscosity decreased in different particle size after germination. Grinding processing improved the viscosity and enthalpy, increased the proportion of starch in the whole grain, enhance the brightness, yellowness and redness but the thermal stability decreased in different particle size. The change of nutrients content affected the physicochemical parameters. Correlation analysis showed that the viscosity, flour color and starch gel temperature of BRF and GEF were mainly affected by starch, while that of POF were mainly affected by amino acids. Research on processing technology and particle size of foxtail millet was guidance for selection of raw materials food processing and food formula.