The invention relates to mixed dihydric alcohol modified copolyester. The mixed dihydric alcohol modified copolyester is characterized by being prepared from the following components of dicarboxylic acid and mixed dihydric alcohol according to a molar ratio of 1:(1.04 to 2), wherein the dicarboxylic acid adopts terephthalic acid PTA; the mixed dihydric alcohol consists of ethylene glycol MEG, 1,2-propanediol PDO, and diethylene glycol DEG; 1,2-propanediol PDO accounts for 1 to 10mol% of total usage amount of mixed dihydric alcohol; the usage amount of diethylene glycol DEG is no more than 6mol% of total usage amount of mixed dihydric alcohol. The invention also relates to a preparation method of the mixed dihydric alcohol modified copolyester. Compared with the prior art, the mixed dihydric alcohol modified copolyester has the advantages that the preparation method is reasonable; the viscosity is high, the transparency is high, the content of acetaldehyde is low, and the fluidity of polymer is good.