The invention discloses a method for preparing an indium phosphide crystal by using an indium-phosphorus mixture, belongs to the technical field of semiconductors, and comprises the steps of preparing an indium-phosphorus mixed ball, charging, maintaining the high furnace pressure and the low temperature of the indium-phosphorus mixed ball, melting a covering agent, feeding, synthesizing and crystal growing, which is synthesized by directly melting the proportioned indium-phosphorus mixed ball. Indium powder and phosphorus powder are uniformly mixed and pressed into spherical indium-phosphorus mixed particles, then the mixture of the indium-phosphorus mixed balls and the boron oxide powder is fed into a melt with a boron oxide covering agent, and crystal growth in situ is performed after synthesis. The method has the advantages of short reaction time, high efficiency and raw material saving, which can effectively reduce the risk of contamination of materials, saves procedures and reduces the material preparation cost.