Fly ash (FA) usually has an insufficient reaction degree in cement-based materials, and the quantification of its reaction degree is essential for understanding its pozzolanic hydration behavior. This work presents a new approach to quantifying the reaction degree of FA in blended cement systems, based on the chemical composition differences revealed by image analysis. We considered some specific elements rich in the unreacted FA particles but poor in the surrounding paste as the characteristic elements, and then employed them to segment the unreacted FA particles from the blended cement paste. In light of the high heterogeneity of FA, one can conduct intensity normalization during the analysis, using grey-level images that are displayed in the intensity of each characteristic element, respectively. At last, the unreacted FA particles, including the partially reacted and irregular-shaped ones, were discriminated from the paste, enabling an accurate calculation of the FA's reaction degree.