The invention provides strontium titanate powder and relates to a method of synthesizing powder for a grain boundary layer ceramic capacitor by means of a hydrothermal method. The method mainly comprises the following steps: (1) preparing spherical metatitanic acid; (2) preparing spherical niobium-doped strontium titanate; (3) semi-conducting the niobium-doped strontium titanate; (4) wrapping metatitanic acid; and (5) converting metatitanic acid into strontium titanate on the surface. By adopting the two-step hydrothermal method, core-shell structure powder of insulative strontium titanate coated semi-conducted strontium titanate is prepared, so that the spherical shape of the core-shell structure powder is finally synthesized, and meanwhile, high-performance powder is provided for preparing the grain boundary layer ceramic capacitor.