The invention discloses a method for increasing grease accumulation of Monoraphidium on the basis of waste molasses mash, and belongs to the technical field of biodiesel. The method comprises the following steps: the waste molasses mash is diluted to serve as an induction medium for Monoraphidium sp.FXY-10, pH is regulated to be 6.8-7.0, and subpackaging and sterilization are performed; well cultured seeds are inoculated to the medium, melatonin with different concentrations is added, and illumination shaking culture is performed; algae cells are collected centrifugally each day, frozen and ground, then grease in the Monoraphidium is extracted with an organic solvent, and the biodiesel is prepared. The method is simple and feasible to operate, production cost is reduced, grease accumulation can be increased effectively by the waste molasses mash under the action of an inducer, and environmental pollution in the waste molasses mash treating process can be reduced.