Abstract: Nano xonotlite fibers were synthesized by hydrothermal method in this study. The effects of lime activity and silica powder particle size on the hydrothermal synthesis of xonotlite were studied, and the phase and microstructure of xonotlite fibers were characterized by x-ray diffractometer (XRD) and field emission scanning electron microscopy (FESEM). The results show that the activity of lime obtained by calcination for 4 hours at 1 000 ℃ is higher than other temperatures. The lime combined with 23 μm silica powder and ZrOCl₂ as additive can synthesize nano-scale xonotlite fibers with diameter 82 nm and bulk density 70.4 kg/m³. The results can provide theoretical guidance for the industrial production of ultra-light xonotlite.