The invention relates to a method for increasing grease content of Monoraphidium on the basis of waste molasses mash and metal ions, 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 the Monoraphidium, then MgSO4.7H2O is added, pH is regulated to be 6.8-7.0, and subpackaging and sterilization are performed; Monoraphidium seeds well cultured by mixotrophic BG-11 are inoculated to the medium, 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 content can be increased by the waste molasses mash, and environmental pollution in the waste molasses mash treating process can be reduced.