Copper slag dilution is an important method for the sustainable development of copper metallurgy industry. As high-quality copper concentrates continue to be consumed, low-grade, high-impurity copper concentrates are gradually being utilized. As a typical low-quality copper concentrate, the content of Al2O3 in the copper slag after smelting of high-alumina copper concentrate has increased. With the change in copper slag composition, controlling the slag characteristics is critical to recover copper in the copper slag dilution process and to reduce copper losses in slag. In this work, ISASMELT copper slag was used as raw material. Analytical techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM-EDS), and infrared spectroscopy (FTIR) are used to elucidate the effects of different Al2O3 contents on the phase and structure of copper slag during the copper slag dilution, as well as the effect of phase transformation and structure change on copper loss in slag. The effects of different CaO additions on the phase transformation, structure and copper loss of high-alumina copper slag are also investigated. The results show that with the increase of Al2O3 content, high melting point hercynite can be formed in the slag, and the polymerization of slag increases and the aluminosilicate structure becomes more complex. This increases the slag viscosity and the copper loss of slag. The addition of CaO in the high alumina copper slag would form high melting point hedenbergite, monticellite, and the viscosity increases. However, with the increase of CaO addition, the slag structure tends to simplify and viscosity decreases. At the CaO addition of 2wt%, the slag contains the lowest copper content of 0.78wt%. The addition of small amount of CaO can reduce the copper loss of high-alumina copper slag.