The invention relates to a polyether ester/mineral oil composite defoaming agent and a preparation method thereof. The method relates to the field of fine chemicals, and especially relates to the preparation of the polyether ester/mineral oil composite defoaming agent for a water-based paint. The polyether ester/mineral oil composite defoaming agent comprises, by weight, 10-20 parts of polyether ester, 19-37 parts of modified mineral oil, 1-10 parts of a defoaming aid and 2-14 parts of an emulsifier. According to the polyether ester, polyethylene glycol and polypropylene glycol are subjected to esterification connection with dibasic acid, and residual hydroxyl groups are subjected to end-capping with fatty acid. The polyether ester/mineral oil composite defoaming agent has the advantages of simple preparation process and low cost. The prepared defoaming agent meets the requirements of water-based paints.