A hydrothermal route for preparing large size xonotlite fibers without special instruments and synthesis conditions was development. Effect of pH values in system on the crystal phase and morphology of synthesized xonotlite fibers was studied by using XRD and SEM. The results indicated that the different pH systems have little effect on the crystallinity of synthesized xonotlite, but have great impact on the morphology of xonotlite fibers. The higher pH value is, the lager scale xonotlite fibers appear when the pH values are among 12.0, 12.5, 13.0 and 13.5. The formation of xonotlite fibers was promoted and xonotlite fibers with 10-30 μm in length and 100-200 nm in diameter were prepared at 220 °C for 15 h in the system of pH 13.5. The evaluation of thermal property revealed that synthesized fibers were excellent in thermal stability at high temperature, and could be potential candidate as reinforcement reagents in composite areas.