The invention relates to a preparation method of composite electrolyte powder for a medium-and-low-temperature solid oxide fuel cell. The method is characterized in that the method comprises the steps that spherical SDC nano powder is prepared on a high temperature and high pressure condition by a hydrothermal method; then the SDC powder is added to a prepared metal-nitrate mixed solution; carbamide is added to serve as a homogeneous precipitation agent; oil bath treatment is conducted at 85 DEG C until the solution is evaporated to dryness and becomes pasty; a paste is calcined at 800-1000 DEG C; and the SDC based composite electrolyte powder is produced.