Abstract: Based on the liquid-liquid phase segregation theory for monotectic alloys, this paper deals with the liquid-liquid phase segregation in syntectic compound melt and the absorption of secondary droplets on the solid-liquid interface. Growth theory of syntectic compound single crystal from melt is proposed, which includes the methods for suppressing the coalescence of secondary droplets and for enhancing the absorption of secondary droplets on the solid-liquid interface, and the principle for controlling the stoichiometry of crystal.