Abstract: Synthesis of calcium hydroaluminate of hexagonal structure was reported. The property of calcium hydroaluminate was examined by analysis of chemistry, XRD and TG-DTA. The experimental results showed that the synthetical sample comprise a little calcium hydroaluminate of cubic structure and calcium hydroxide besides calcium hydroaluminate of hexagonal structure. The content of SiO2 decreased to 0.009 g/L and the silicon capacity index reached 11111 by adding calcium hydroaluminate of hexagonal structure at 90°C after two hours. The desilication mechanism of calcium hydroaluminate of hexagonal structure was discussed. It was regarded that calcium hydroaluminate of hexagonal structure was a high-hydration, high-decentralization and loosen calcic compound. Its interval of layers was wide. Since the desilication reaction processed in the interval, the efficiency of desilication was high.