To recover copper from copper slag, a novel slag cleaning method was proposed. The thermodynamics and binary phase diagram of FeO-SiO₂ were calculated first. Then, the effects of experimental parameters on the reduction of Fe₃O₄, recovery of copper, and slag cleaning mechanism were investigated. The results showed that the addition of SiO₂ significantly reduced the initial reduction temperature of magnetite and pyrite. The contents of Fe₃O₄ and copper decreased to 2.45 wt.% and 0.39 wt.%, respectively, with copper recovery of 92.52% under the optimum conditions. More than 94% silver and 99% gold subsided into the copper matte after slag cleaning. Approximately 72% of zinc and 75% of lead volatilized in the flue gas. The viscosity of the copper slag decreased from 0.68 to 0.12 Pa·s due to the reduction of magnetite. The sedimentation of copper matte droplets could be enhanced by mechanical stirring.