In this paper, a non-toxic and low-energy-consuming process for in-situ growth of mullite whiskers on the surface of aluminum silicate fiber was developed. Firstly, the thermophysical properties and morphology of mullite whisker reinforced aluminum silicate fiber composites were characterized, and the optimum heat treatment temperature for in-situ growth of mullite whiskers under borax fluxing was determined. Then, under the optimum heat treatment temperature, silica sol and aluminum nitrate were used as silicon source and aluminum source respectively, and borax was used as flux. The effects of flux concentration and holding time on whisker morphology, phase composition, density and compressive strength were analyzed. The results show that mullite whiskers with “thin needle shape” are successfully grown on the surface of aluminum silicate fiber under borax fluxing. The optimum conditions are as follows: heat treatment temperature 900 ℃, holding time 2 h, flux concentration 0.17 mol/L, silica sol concentration 0.17 mol/L, aluminum nitrate concentration 0.51 mol/L. The compressive deformation of mullite whisker reinforced aluminum silicate fiber can reach up to 60%, and the compression strength of this material can reach up to 0.462 MPa at 60% compression deformation.