A water-soluble salt core (WSSC) strengthened by reinforcing particles, including bauxite powder, glass fiber powder, and sericite powder, was fabricated by gravity-casting process. The surface quality, bending strength, water solubility, humidity resistance, and shrinkage rate of WSSC were investigated, and the synergistic effect between the different reinforcements on the bending strength was analyzed. Scanning electron microscope (SEM) was used to study the micromorphology of WSSC. The results indicate that the binary composite WSSC after being strengthened has excellent comprehensive performance, the bending strength increases by more than 1.4 times with the maximum value of 47.89 ± 0.83 MPa whose 24-h hygroscopic coefficient is lower than 0.18%, the water solubility rate is higher than 163.97 kg/(min m3) in still water at 80 °C, and the shrinkage rate is dramatically lower than that without any reinforced materials; in addition, there are no obvious casting defects on the core surface. The microscopic analysis demonstrates that the homogeneous distribution of the reinforcements in the matrix consumes more energy during the crack propagation procedure and the grain refinement of WSSC is also observed, above which is the main reason for the improvement of the bending strength. Furthermore, the practical casting test of the complex soluble salt core prepared by pressure core making was used for zinc alloy die casting.