The glass-ceramics of CaO-MgO-Al2O3-SiO2-Cr2O3 system was prepared by melting method using blast furnace slag, low-carbon ferrochrome alloy slag and quartz sand as raw materials, and the effect of Cr2O3 on the viscosity and structure of slag (or glass) of CaO-MgO-Al2O3-SiO2 (CMAS) system at high temperature was studied. The Urbain viscosity prediction model was established and optimized, and the effect of Cr2O3 on the structure of slag (or glass) was studied by Raman spectroscopy. The results show that when the mass fraction of Cr2O3 is in the range of 0.85–2.05%, the viscosity of slag (or glass) of CMAS system decreases with the increase of Cr2O3 content. The average relative errors between the experimented viscosity value and the calculated viscosity value obtained by using the optimized Urbain model are less than 20%, which is effective and universal for the viscosity prediction of slag (or glass) of CaO-MgO-Al2O3-SiO2-Cr2O3 system. With the increase of Cr2O3 content, the complex silicon oxygen tetrahedrons (Q3) disintegrate into a larger number of simple silicon oxygen tetrahedrons (Q0, Q1, Q2), resulting in the sparse structure of the melt network and a decrease in macroscopic viscosity.