Abstract: The coagulation process is a widely applied technology in water and wastewater treatment. Novel composite polyferric magnesium–silicate–sulfate (PFMS) coagulants were synthesized using Na₂SiO₃·9H₂O, Fe₂(SO₄)₃, and MgSO₄ as raw materials in this paper. The effects of aging time, Fe:Si:Mg, and OH:M molar ratios (M represents the metal ions) on the coagulation performance of the as-prepared PFMS were systematically investigated to obtain optimum coagulants. The results showed that PFMS coagulant exhibited good coagulation properties in the treatment of simulated humic acid–kaolin surface water and reactive dye wastewater. When the molar ratio was controlled at Fe:Si:Mg = 2:2:1 and OH:M = 0.32, the obtained PFMS presented excellent stability and a high coagulation efficiency. The removal efficiency of ultraviolet UV₂₅₄ was 99.81%, and the residual turbidity of the surface water reached 0.56 NTU at a dosage of 30 mg·L^–1. After standing the coagulant for 120 d in the laboratory, the removal efficiency of UV₂₅₄ and residual turbidity of the surface water were 88.12% and 0.68 NTU, respectively, which accord with the surface water treatment requirements. In addition, the coagulation performance in the treatment of reactive dye wastewater was greatly improved by combining the advantages of magnesium and iron salts. Compared with polyferric silicate–sulfate (PFS) and polymagnesium silicate–sulfate (PMS), the PFMS coagulant played a better decolorization role within the pH range of 7–13.