Copper slag is a solid waste generated during the pyrometallurgical process. The iron mass fraction is about 40%. The amount of copper slag discharged in China is as high as 14 million tons per year. Therefore, it is of great significance to study the iron extraction from copper slag for comprehensive utilization of resources. Combined with the phase distribution of copper slag, the promotion and inhibition mechanisms of calcium oxide in the reduction process of copper slag were introduced. The promotion effects were that calcium oxide can significantly reduce the initial reaction temperature of copper slag reduction system and destroy the structure of iron olivine in copper slag, promote the release of ferrous oxide in the slag phase, reduce the nucleation barrier of iron grains, and accelerate the nucleation of iron grains. The inhibition effect was that calcium oxide and silicon oxide formed high melting point calcium silicate salt, which increased the solid phase ratio of the copper slag phase and hindered the aggregation and growth of iron grains.