The resourceful utilization of chelated fly ash in road engineering can reduce the occupation of land resources, but there is no consensus of its feasibility as subgrade filler. Through indoor experiments and microscopic analysis, the physicochemical properties of chelated fly ash, as well as the influence of moisture content and dry density on its engineering characteristics, were investigated in this study. The feasibility of employing chelated fly ash as subgrade filler in highway construction was explored, accompanied by proposed construction control indicators and measures to prevent environmental pollution. The results show that: (1) Chelated fly ash is classified as a type of high liquid limit silt featuring a complex microstructure and low specific gravity, with a higher optimum moisture content and lower maximum dry density compared to common fillers. (2) The primary chemical elements in chelated fly ash are Ca, Na, Si, and Cl, with major mineral components including calcium compounds, silicon dioxide, and chlorides. Calcium compounds can solidify fly ash by chemical precipitation. (3) Strength, effective cohesion, and effective internal friction angle decrease approximately linearly with increasing moisture content, and increase with increasing dry density, while permeability coefficient decreases with increasing dry density. (4) Chelated fly ash is suitable for highway subgrade filler, with recommended moisture content ranges of 22.0% to 29.5% for embankments and 24.5% to 29.5% for roadbed materials. To ensure subgrade stability and reduce environmental pollution risks, it is recommended to employ low liquid limit clay cover and waterproof geotextiles core-wrapping for combined treatment. This study provides support for determining the technical parameters of chelated fly ash subgrade filler, and holds significant engineering value for its resource utilization in road engineering.