The invention discloses a magnesium phosphate cement-based composite material property modifying additive. The additive comprises components in percentage by weight as follows: 9.8%-17.9% of an expanding agent, 4.9%-9.0% of a shrinkage reducing agent, 12.8%-23.3% of organic fibers, 1.4%-5.0% of a fiber dispersing agent, 20.1%-33.0% of a viscosity modifier, 12.2%-33.5% of a thickening time control agent and 4.4%-12.5% of a superplasticizer. The magnesium phosphate cement-based composite material property modifying additive comprises the simple components, the preparation technology is simple and rapid, the environment applicability is high, the usage method can be flexibly chosen according to construction site equipment and technology conditions, and construction operation and efficiency improvement are facilitated; a magnesium phosphate cement-based composite material prepared from the additive has the characteristics of wide thickening time control range, high mechanical property, good volume stability, excellent bonding property, good durability and the like, service performance of magnesium phosphate cement repair materials and reinforcement materials are greatly improved, and application and popularization of the 3D printing technology in the building field are particularly facilitated.