In order to reduce the pollutant loading of bio-ecological units in the biogas slurry treatment process, the magnetic coagulation process was adopted to pre-treat the biogas slurry in this paper. The dosage of magnetic seeds and coagulations was first optimized by single-factor and response surface experiments, and then process performance and mechanism were explored by analyzing Al content and Al species of coagulation effluent and the distribution of organic functional groups from coagulation flocs. The results showed that magnetic coagulation accelerated the settling of flocs, improved the safety of effluent, and strengthened the removal of pollutants. The optimal coagulant composition for magnetic coagulation was Fe₃O₄(1.0 g·L^-1)+Al₂(SO₄)₃(2.05 g·L^-1)+PAM (10.34mg·L^-1), and the removal efficiency of PO₄^3--P, COD, and NH₄⁺-N respectively reached 99.09%, 30.39%, and 14.76%, at this level. At the same time, the removal of organic matter was enhanced at magnetic coagulation in biogas slurry by promoting Al_b species generation and improving aromatic carbon compounds removal. Besides, the magnetic adsorption improved NH₄⁺-N removal during coagulation. The results provide technical references for biogas slurry pretreatment.