The invention relates to a compound additive for biodiesel and a preparation method thereof, belonging to the field of diesel additives. According to the compound additive for biodiesel, provided by the invention, diesel is added, so that the solidification temperature of the biodiesel can be reduced and the biodiesel has a good low-temperature flowing property, and therefore, the diesel is convenient to store, convey and use. The preparation process of the product is simple, convenient to operate and low in cost. By virtue of a synergistic effect among the components, the compound additive can be used for simultaneously improving various performances of biodiesel.