The xonotlite fibers were synthesized via the hydrothermal synthesis method with CaO and SiO2 as the raw materials and the molar ratio of Si/Ca of 1.0. Effect of anions from various calcium sources on the microstructure of the xonotlite fibers is studied in this paper. These obtained products were characterized by X-ray diffraction (XRD), transmission electron microscope (TEM) and scanning electron microscope (SEM) techniques to investigate their crystalline phase, crystal structure and morphology. The results indicate that anion from various calcium sources has little influence on the crystalline phases of xonotlite fibers but poses a great impact on their morphologies. Xonotlite fibers with single crystal characteristics and large aspect ratio of 50—100 were successfully fabricated from CaCl2 as calcium material at 225 °C for 15 h. The existence of Cl− anion in the CaO-SiO2-H2O system significantly contributes to the formation of xonotlite crystal.