Abstract: The harmless disposal of phosphogypsum (PG) and the development of novel building materials constitute significant research topics within the field of environmental engineering. Consequently, this study systematically investigates the effects of PG on the mechanical properties, durability, and environmental safety of ground granulated blast furnace slag (GGBFS) and steel slag (SS)-based concrete (PCM). It further elucidates the mechanism of action of PG within the PCM cementitious system and the potential P/F solidification mechanism. Results indicated that in the optimal concrete mix design P3, the mass fractions of PG, GGBFS, SS, and calcium oxide (CaO) were 25%, 37.5%, 37.5%, and 1%, respectively. C3 achieved a compressive strength of 32.75 MPa at 90 days, with an elastic modulus of 3.39×10⁴ MPa and electric flux as low as 141 C, fully meeting the mechanical and durability requirements for C30 concrete. A positive correlation was observed among the macro compressive strength, elastic modulus, and resistance to chloride ion permeation of PCM. In terms of environmental safety, the PCM system demonstrated exceptional immobilization capabilities for phosphorus and fluorine. The leaching concentrations of phosphorus and fluorine decreased from initial levels of 11.921 and 8.340 mg/L to 0.055–0.124 and 0.027–0.104 mg/L, respectively, while heavy metal leaching concentrations fell below the detection limit. The hydration mechanism indicates that within an alkaline cementitious system, the free phosphorus/fluorine-containing anions and \mathrmSO_4^2- ions released from phosphogypsum dissolution may undergo a competitive reaction. This promotes the participation of \mathrmSO_4^2- ions in the formation of ettringite (AFt). Concurrently, an appropriate amount of PG can synergistically induce, together with CaO, the disruption of the glassy silica-alumina chain structure within GGBFS, accelerating the release of active Si and Al tetrahedral units and thereby enhancing the polymerization degree of the C—(A)—S—H gel. Highly crystalline AFt and C—(A)—S—H gel jointly form a dense microstructure. However, excessive PG (50%) inhibits the hydration reaction.