The invention discloses a self-expanding composite bone cement and a preparation method thereof. The solid phase component and the liquid phase component are mixed according to the mass volume ratio of 1g:1.5ml~1g:7.2ml; according to the mass percentage, the raw material composition of the solid phase For: polymethyl methacrylate-acrylic acid-anhydrous calcium hydrogen phosphate copolymer microspheres 30% to 70%, anhydrous calcium hydrogen phosphate 15% to 35%, tetracalcium phosphate 15% to 35%, the above components The sum of the mass percentages is 100%; the bone cement liquid phase is any one of distilled water, blood, physiological saline, dilute acid, serum, and phosphate solution. The self-expanding composite bone cement prepared by the invention has high water absorption expansion ratio and good mechanical properties, and is expected to become a promising bone cement for clinical application.