The invention discloses a novel method for preparing algae environment-friendly diesel by virtue of a one-pot process and belongs to the technical field of new energy sources. The novel method aims at solving a lot of disadvantages of preparation of high-quality algae biofuel by virtue of a two-step process. The novel method is characterized by comprising the following steps: firstly, weighting and placing a certain amount of algae powder, a precious metal catalyst and waste engine oil in a high-pressure reactor, sealing and replacing air in the reactor with hydrogen with a certain pressure; and secondly, placing the reactor in a salt bath with a certain temperature and reacting for a period of time; after the reaction is completed, cooling the reactor to room temperature, reducing the pressure and opening the reactor, extracting with an organic solvent, filtering to remove residues to obtain a liquid mixture and carrying out spin evaporation on the liquid mixture to remove the organic solvent to obtain the environmental-friendly diesel. The yield of the environmental-friendly diesel is 60-80wt%, the main component is a hydrocarbon, the content of oxygen is 0.3-2.6 wt%, the calorific value is 44-50MJ/kg, the molar ratio of H to C is 1.8-2.0 and the content of sulfur reaches the national IV standard of automotive diesel.