The invention discloses a compound diesel fuel additive and a preparation method. The additive is prepared from, by weight, 5-15 parts of functional groups with cleaning performance, 15-25 parts of functional groups for energy conservation and emission reduction, 3-6 parts of functional groups for lubrication and 55-70 parts of diluent. The compound diesel fuel additive can effectively improve the quality of diesel fuel, diesel fuel can be saved by 10%-30% under the same operating condition of a diesel engine, the free acceleration smoke opacity index can be reduced by 90% or above, the effects of removing carbon deposit and inhibiting carbon deposit generation can be achieved, functional combination is achieved, and the compound diesel fuel additive has the advantages of being excellent in cleaning performance, capable of efficiently saving fuel and reducing tail gas emission and the like. According to the preparation method of the compound diesel fuel additive, the process is easy and convenient to implement, energy consumption is small, compatibility and compounding of all the functional groups can be effectively achieved, and the prepared additive is stable and can be safely transported.