The invention discloses amorphous copolyester and a preparation method thereof. The preparation method of the amorphous copolyester comprises the following steps: (1) in an inert atmosphere, carrying out an esterification reaction of at least two diacids and at least two diols under a condition of the presence of a catalyst; and (2) adding a stabilizer and a catalyst into the system after the esterification reaction in the step (1), and carrying out a polycondensation reaction under a vacuum condition to obtain the amorphous copolyester. The amorphous copolyester has the characteristic of low molding temperature; the amorphous copolyester has no melting point, the molding temperature is decided by the softening temperature, and the highest molding temperature is only about 100 DEG C. The amorphous copolyester has the characteristics of small product shrinkage percentage and good dimensional stability; and since the copolyester is amorphous, no product shrinkage problem exists, and the product dimensional stability is good.