Effect of anion impurity on preparing xonotlite whiskers via hydrothermal synthesis was researched in this paper. The crystal phase composition and morphology of the xonotlite whiskers were analyzed using XRD and techniques. The results indicate that different anion impurity have little effect on the crystal phase composition of synthesized the products, but have great impact on the crystallinity and morphology of xonotlite whiskers. Xonotlite whiskers with well crystal and aspect ratio of 50-100 were successfully prepared from CaCl2 as calcium sources under the condition of 225 °C for 15 h with the reactant concentration of 0.05mol•L-1, but the hydrothermal process condition should be controlled at 225 °C for 40 h while using Ca(NO3)2 or Ca(OH)2 as calcium sources. Cl- iron can accelerate the process of hydrothermal synthesis and be beneficial to the crystallization of xonotlite in CaO-SiO2-H2O system.