The invention discloses a preparation method for a mullite whisker with high length-diameter ratio by in-situ growth, and relates to a preparation method for the mullite whisker. The preparation method provided by the invention is to solve the technical problems of the need of firing at a high temperature (larger than 900 DEG C) and low length-diameter ratio for a majority of conventional mullitewhisker preparation methods. The preparation method provided by the invention comprises the following steps: 1, preparation of an alumina sol; 2, preparation of a silica sol; 3, hydrothermal treatment; 4, freezing and freeze-drying; and 5, heat treatment. According to the invention, a nanoscale fibrous substance can be formed in a hydrothermal system, and energy required by growth of the mullite whisker in the process of heat treatment can be reduced, so the preparation temperature of the mullite whisker is reduced; the mullite whisker prepared by hydrothermal treatment and heat treatment canbe used for preparing the mullite whisker with high length-diameter ratio at a low reaction temperature; and the preparation method provided by the invention has the advantages of low reaction temperature, good repeatability, high yield and the like.