The speciation of heavy metals is crucial for their bioavailability, environmental toxicity, and other aspects. The improved BCR method was used to study the speciation of heavy metals in the neutralized slag and loess in the soil matrix in an ecological restoration demonstration site of a copper mine. The results show that the total contents of heavy metals, such as Cu, Zn, As, Pb, Cr, Ni, and Cd, in the neutralized slag and loess in the soil matrix in the ecological restoration demonstration site of the copper mine all meet the second-class land control values specified in GB 36600-2018 Soil Environmental Quality Risk Control Standard for Soil Contamination of Development Land (Trial). The speciation of heavy metals in the neutralized slag and loess is predominantly in the residual fraction, which is highly stable, weakly mobile, and poses minimal potential environmental hazards. The stability of the heavy metals in the neutralized slag indicates its reliable environmental safety as a soil matrix. Using neutralized slag blended with natural soil for ecological restoration and phytoremediation is feasible.