The invention discloses a preparation method of stearate hyperbranched polyether grafted sisal microcrystals. The method comprises the following steps: weighing sisal microcrystals, tetrahydrofuran and potassium methoxide, sequentially adding into a three-neck flask, heating to react for 1 hour under the protection of N2, adding dioxane and glycide, heating to react for 24-26 hours, adding methanol to terminate the reaction, pouring the reaction product into an organic solvent to precipitate, filtering, and carrying out vacuum drying for 24 hours to obtain hydroxyl-terminated hyperbranched polyether grafted sisal microcrystals; weighing the hydroxyl-terminated hyperbranched polyether grafted sisal microcrystals, stearic acid and a catalyst, adding into a three-neck flask, and heating and stirring under reflux to react for 11-13 hours under the protection of N2; and after the reaction finishes, pouring the product into a beaker filled with an organic solvent, carrying out precipitation separation, filtering, and carrying out vacuum drying to obtain the stearate hyperbranched polyether grafted sisal microcrystals. The method has the advantages of abundant raw material, low price, small environmental pollution and simple preparation steps, and is convenient for large-scale production. The product can effectively toughen and modify the wood-plastic composite material.