The invention discloses a composite scale inhibitor, comprising sodium polyaspartate and hydroxy-ethylidene-di-phosphonic acid in the mass ratio of 1:20-1:1. The composite scale inhibitor can be prepared by mixing the sodium polyaspartate with the hydroxy-ethylidene-di-phosphonic acid according to the mixture ratio, adjusting the pH value to be 1-5 and uniformly stirring. The composite scale inhibitor is applied to equipment for preparing salts and is especially suitable for mirabilite type brine vacuum equipment for preparing salt; and when the composite scale inhibitor is applied, the addition of refined brine water is 5-25mg/L. The binary composite scale inhibitor has favorable scale inhibiting effect and corrosion inhibiting effect, thereby effectively preventing the system and the equipment for preparing salts from being scaled due to the continuous enrichment of calcium and magnesium ions in the mirabilite type brine water; and the scale inhibiting rate is over 94 percent. Whenin use, the composite scale inhibitor is simple for preparation, convenient for operation and relatively low in cost, so that the composite scale inhibitor has favorable practicability and the use value.