The order of reactions for Mg-B₂O₃-TiO₂ system was determined by thermodynamic calculation. Then the composition and morphology evolution of the product prepared by self-propagating high temperature synthesis were analyzed in terms of reaction zones of the process by Cu wedge combustion wave quenching method. The formation and growth mechanism of the TiB₂ crystal grains was investigated as well. The results of thermodynamic calculation show that in the process of SHS reaction B and MgO were firstly obtained by reduction reaction between Mg and B₂O₃, then Ti and MgO was obtained by reduction reaction between Mg and TiO₂; finally B reacts with Ti to form TiB_2. In this process, the formation possibility of the intermediate products decreases corresponding to the following order: Ti₃O₅、Ti₂O₃ and TiO. The experimental results show that no intermediate products may be detected in the combustion center, where the reaction was entirely complete; however in zones near the center or at the edge there existed a small amount of Ti₂O₃ and TiO, where temperature was not high enough for completing the reaction; at the bottom zone of the combustion there existed a little Ti₃O₅, where temperature was too low for the reaction. Therefore, thermodynamic prediction coincides well with experimental results. It follows that during the reaction process of SHS, MgO firstly nucleates and grows up, while TiB₂ may form through tow ways, by one way TiB₂ nucleates on MgO crystals, and then grows into tiny particles as the rising temperature; by the other way TiB₂ independently nucleates and grows up into hexagonal crystal in between large MgO crystals. The growth of TiB₂ follows typical L-S mechanism; B and Ti alternatively gather and grow up to form hexagonal crystal.