A method for preparing biodiesel includes the steps of: by using larvae or pupae of complete metamorphosis insect as raw materials, extracting insect oil by solvent process, conducting transesterification or pre-esterification-transesterification respectively of the low-acid-value (acid value≦2) insect oil or the high-acid-value (acid value>2) insect oil to get crude biodiesel after stratification; and purifying and processing the crude biodiesel by using adsorption clarification technic, molecular distillation techinic, and freezing (low temperature) separation technic to get refined biodiesel. The advantages of the method include wide variety of raw materials, easy to breeding, and low cost of production. The present invention provides a new technical way of producing biodiesel, and on the other hand, it also provides a new way of exploiting insects.