The synthesis of (Ca, Mg)α′-Sialon ((Ca, Mg)xSi12-3xAl3xOxN16-x) powders using titania-bearing blast furnace slag as a starting material by carbothermal reduction–nitridation (CRN) was reported for the first time. The reaction processes were greatly affected by initial material and reaction parameters. With the compositions shifting range from x = 0.3 up to 2.0, the amount of (Ca, Mg)α′-Sialon, TiN and AlN, the solid solubility of Ca2+ and Mg2+, the unit cell parameters of the (Ca, Mg)α′-Sialon and the amount of elongated α′-Sialon grains increased, but β′-Sialon and SiC decreased. With the increase of synthesis temperature and holding time, the formation of (Ca, Mg) α′-Sialon in the products increased and the CRN reactions accelerated. The optimum synthesis conditions of (Ca, Mg) α′-Sialon were 1480℃ for 8 h, under which the crystalline phases of the products also included AlN, TiN and a small amount of SiC and β-CaSiO3. The volatilization of SiO resulted in a mass loss of samples, which was enhanced with the increase of synthesis temperature and holding time.