Abstract: To address the urgent need for the resource utilization of waste steel slag and the difficulty in the operation and maintenance of chemical phosphorus removal in rural domestic sewage treatment, an alkali-modified steel slag phosphorus removal fillers were prepared using an orthogonal experimental design. The surface characteristics of the fillers were characterized using Scanning Electron Microscopy (SEM), X-ray Diffraction (XRD), and Fourier Transform Infrared Spectroscopy (FTIR). An alkali-modified steel slag filter column was set up to treat the effluent from a rural domestic sewage biological treatment unit, and the influences of hydraulic retention time (HRT) and filler particle size on the phosphorus removal performance were mainly investigated. The results indicated that the rough and porous surface structure of the fillers provides abundant physical sites for phosphorus adsorption, and the crystal structure remains stable without significant phase transformation after adsorption. The adsorption mechanism involves chemical precipitation by active components such as calcium oxide and ferric oxide, as well as coordination between silicon dioxide surface functional groups and phosphorus. The decrease in particle size (15–20 mm, 10–15 mm, 5–10 mm, and 3–5 mm) of fillers could increase their specific surface area and active site density, and the total phosphorus (TP) removal (above 80%) of the column was the highest with 3-5 mm particle size filler, followed that with 5-10 mm particle size filler (57.5% TP removal efficiency). The 5–10 mm filler was recommended as the optimum filler taking both‌ the mechanical strength and clogging risks into account. The phosphorus removal of the column was significantly improved with the HRT increase for treatment of the effluent from a rural domestic sewage biological treatment unit , and the TP removal efficiency could achieve 99% at 5.0 h HRT. Considering operational costs and TP removal efficiency, 2.5 h was determined as the optimal HRT. The column effluent TP concentration was lower than 0.3 mg/L, which met the Class-I discharge limitation value (0.5 mg/L) of Hebei Province's Rural Domestic Sewage Discharge Standard (DB13/2171-2020). The laboratory-scale preparation cost was estimated at 10.13 ¥/kg, and the cost of full-scale was expected to decline substantially, while the process inherently embodied the circular economy principle of "treating waste with waste" and exhibited considerable potential for engineering application. In summary, the alkali-modified steel slag composite filler exhibited excellent engineering application potential, offering a theoretical foundation for steel slag resourceful utilization and efficient phosphorus removal in rural wastewater treatment.