The invention discloses an ultralow-shrinkage composite resin and a preparation method thereof. The ultralow-shrinkage composite resin is formed by compounding a resin matrix and inorganic filler. The resin matrix comprises a monomer system and an igniter system which are polymerizable. The inorganic filler comprises inorganic filler obtained after surface silylanization and calcium silicon-phosphate powder. Artificial bone cement-calcium silicon-phosphate are applied to a dental composite resin material, the calcium silicon-phosphate powder suffers from volume expansion after being subjected to a hydration reaction, compensation for polymerization volume shrinkage of the composite resin can be effectively achieved, the volume stability of the composite resin can be improved, and meanwhile mechanical indexes of the composite resin can meet the requirements of clinical dental materials. In addition, the preparation process is simple, cost is low, and important popularization and application value is achieved. The calcium silicon-phosphate filler has wide application prospects in the field of mouth rehabilitation through the characteristics.