The invention discloses a method for producing biodiesel by using kitchen wastes. The method comprises the following steps: carrying out hydro-thermal treatment on the kitchen wastes in a hydro-thermal reactor; secondly, separating grease from the kitchen wastes after the hydro-thermal treatment; enabling the grease to flow into a primary or multi-level reaction provided with immobilized enzyme; carrying out transesterification by adding short-chain alcohol to prepare the biodiesel. The invention provides a novel method for promoting lipase to catalyze kitchen waste oil to produce the biodiesel through the hydro-thermal pretreatment; the lipase which is simple and easy to obtain is selected and a composite lipase is prepared for effectively catalyzing the transesterification of the kitchen grease, so that the yield of the biodiesel can be up to more than 95%. The quality of the biodiesel meets the requirements of national standards. The method disclosed by the invention is simple and convenient and is particularly applicable to industrial application.