Effect of different blending amount of LaCrO3, a composite additive, on the hydration resistance and crystallization properties of magnesia-calcium refractory were studied by measuring hydration rate, using X-ray diffraction and scanning electron microscopy. The results show that the LaCrO3 at a low addition (≤1%) can significantly decrease the hydration resistance of the MgO-CaO material when the burning temperature is 1550°C, and the hydration rate is only 0.183%. LaCrO3 can promote the growth of the CaO, while it has little effect on MgO. The Cr2O3 and CaO can melt into CaO·Cr2O3 in a high temperature. The CaO·Cr2O3 can not only promote sintering but also wrap on the CaO, which is negative for the grain to contact with water, and consequently improve the hydration resistance of magnesia-calcium refractory.