In order to verify the feasibility of yellow phosphorus slag produced in Yunnan Province as an supplementary cementitious material, this paper compared the grinding and cementitious properties of granulated blast furnace slag and yellow phosphorus slag, and analyzed and compared their properties as supplementary cementitious materials. The results show that granulated blast furnace slag and yellow phosphorus slag have similar chemical composition and mineral phase. After the same grinding time, the particle size distribution, specific surface area and microstructure of granulated blast furnace slag and yellow phosphorus slag are similar. The increase of specific surface area of granulated blast furnace slag and yellow phosphorus slag promotes the hydration of cement and improves the activity of granulated blast furnace slag and yellow phosphorus slag, and the activity of granulated blast furnace slag is higher than that of yellow phosphorus slag under similar specific surface area. The addition of granulated blast furnace slag and yellow phosphorus slag has a great contribution to the later strength of cement mortar. The later strength of cement mortar increases with the increase of specific surface area of granulated blast furnace slag and yellow phosphorus slag. When the specific surface area of granulated blast furnace slag and yellow phosphorus slag is 340 and 521 m2/kg, respectively, the 28 d compressive strength of cement mortar with 30% (mass fraction) granulated blast furnace slag and yellow phosphorus slag is 46.5 and 47.3 MPa, respectively, which is higher than that of pure cement mortar. When the specific surface area of granulated blast furnace slag and yellow phosphorus slag is 775 and 802 m2/kg, the 28 d compressive strength of cement mortar is 58.7 and 50.9 MPa, respectively. In general, yellow phosphorus slag is an available supplementary cementitious material with high activity in this study, but attention should be paid to the retarding effect of yellow phosphorus slag on cement-based materials.