Taking steel slag powder as the object of study, weak acid modified steel slag powder was prepared by phosphoric acid modified steel slag powder. Weak acid modified steel slag powder was characterized by X-ray diffractometer, specific surface area and aperture meter and environmental scanning electron microscope. Composition, pore structure, microstructure and element composition of weak acid modified steel slag powder were researched. The results showed that the proper phosphate solution can effectively remove the f-CaO in the steel slag powder and improve pore structure of weak acid modified steel slag powder. The excess phosphate solution reacts with Ca(OH)2 in steel slag powder, causing structure of weak acid modified steel slag powder to collapse, and showing fleeciness as a whole. When steel slag powder is 80 g and phosphate solution is 1.6~3.2 mL, weak acid modified steel slag powder has low f-CaO content and good pore structure. It provides technical support and theoretical basis for further expanding the utilization methods of steel slag.