The invention discloses a fatty polyester compound and a preparation method thereof, including the following steps: (1) synthesis of condensation fatty co-polyester: the react between two, three or four fatty dicarboxylic acids or the esters thereof and fatty dibastic alcohol is conducted to achieve condensation fatty co-polyester; (2) the condensation fatty co-polyester reacts with poly lactic acid under the existence of double functional-group chain-extension agent and the extruded out, the achieved fatty polyester compound is the blend of high molecular weight fatty co-polymer, high molecular weight fatty block copolymer and high molecular weight poly lactic acid. The weight percents of condensation fatty co-polyester and poly lactic acid drop is 5:95-95:5. The preparation method has the advantages that the condensation duration is short, the process control is stable and simple, and the method can be directly applied to various processing techniques such as film inflation, extrusion, vacuum molding, blow molding, foaming, and the like; the achieved products have high purity and nice color; the performance adjustment is flexible and convenient, and the cost is low.