Excessive discharge of phosphorus in wastewater leads to increasingly serious water pollution. The fly ash was chemically modified into a hydrated calcium silicate adsorbent to explore its absorption effect on the phosphate. XRD, SEM, BET specific surface area and so on were used to characterize the adsorbents before and after particle size classification. Then the adsorption performance of the adsorbents with different particle sizes on phosphate was studied, and the adsorption mechanism was also investigated. The results showed that the chemical compositions of the adsorbents with different particle sizes showed significant segregation, and the pore structures were also significantly different. Specifically, compared to other adsorbent particles, the particles with the size of 50?75 μm had a higher content of calcium and silicon, lower content of aluminum, iron, and magnesium and thereof higher content of hydrated calcium silicate companied with aluminum-containing tobermullite crystals. The increase of calcium ions made it easy to combine with more phosphate to form a precipitate. At the same time, this particle had a higher specific surface area and more porosity and the loose and porous structure provided more active sites for calcium ions. When used as an adsorbent to adsorb phosphate, the saturated phosphorus adsorption capacity of the particles with the size of 50?75 μm can reach 17.1 mg/g, which was 19.58% higher than that of an unclassified adsorbent.