Cadmium Zinc Telluride (CZT) crystal is the substrate material required for fabricating HgCdTe infrared focal plane array detectors. Its preparing technology includes four parts: polycrystal synthesis, crystal growth, substrate processing and quality evaluation. The exothermic process of raw material synthesis and the control method of material stoichiometric ratio are studied. By measuring the temperature of the quartz crucible in the synthesis process, the exothermic rate is calculated. The dynamic process of CZT synthesis reaction is observed by a video surveillance system and the factors which have influence on the synthesis reaction rate are revealed. On this basis, the reaction rate and material stoichiometric ratio are controlled effectively by using the technologies of gradient temperature and directional solidification.