Abstract: Composite calcium ferrite is the main bonding phase in high basicity sinter. As the main element in the sinter, MgO affects not only the formation of calcium ferrite in the sintering process but also its reduction behavior when dissolved in iron ore. This paper studied the effects of MgO on composite calcium ferrite formation and hydrogen-rich reduction behavior. The results show that when the roasting temperature is 1 250 ℃, with the increase of MgO content from 0% to 5.5%, the content of magnesium-containing magnetite increases, which is distributed around or in calcium ferrite, which reduces the liquid phase amount of the sintering process and the rate of needle calcium ferrite production, thus resulting in the decrease of sinter strength and reduction rate. In addition, low-temperature thermogravimetric experiments show that the activation energy of composite calcium ferrite increased at first and then decreased with the increase of magnesium content. When the magnesium content is less than 4%, the most likely reaction mechanism function is R2, a two-dimensional shrinking cylinder phase boundary reaction model, and the differential form of the equation is f(α)= 2(1-α)^1/2.