The invention provides a method for accurately controlling the size of a formed part, which is implemented through the following steps: calculating a size derivation delta1 produced by the elastic recoil of the formed part after demolding, the size derivation delta2 produced by the thermal expansion of a mould, the size derivation delta3 produced by the cooling shrinkage of the formed part, the size derivation delta4 produced by the residual deformation of the mould and the total size derivation delta(total)=delta1+delta2+delta4-delta3 of the formed part respectively. The size of the formed part can be accurately controlled by compensating and correcting the size of a mould cavity because the size compensation and correction of the mould cavity and the total size derivation of the formed part are equal in digital value. The method has the advantages of quickly and conveniently guiding engineering production and improving the quality of the formed part; and engineering verification shows that the size accuracy of the formed part is remarkably improved.