The invention discloses a sludge dewatering flocculant prepared by utilizing bagasse. The sludge dewatering flocculant is prepared from bagasse and montmorillonite in a mass ratio of (85-90):(10-15). A preparation method of the sludge dewatering flocculant comprises the following steps: (1) preparing a mixed liquid: preparing a mixed solution with water, sodium hydroxide and ethanol; (2) pretreating bagasse: adding bagasse to the mixed solution, then putting the solution in an autoclave to be sterilized, filtering the solution, and then drying the filter residue; (3) preparing a bagasse biological flocculant: adding the dried filter residue to the prepared mixed liquid, and adding a compound enzyme to carry out enzymolysis on the filter residue, thus obtaining the bagasse biological flocculant; (4) preparing the sludge dewatering flocculant: adding montmorillonite to the bagasse biological flocculant, and stirring and mixing the materials for a certain time, thus preparing the sludge dewatering flocculant. The sludge dewatering flocculant and the preparation method have the beneficial effects that the cost is relatively low, flocculation effects are good, the production process is simple, and the production cycle is short, thus being beneficial for mass production.