An acoustic insulation device of a transformer is provided. The acoustic insulation device is penetrated by an elevated seat of the transformer. The acoustic insulation device comprises a support frame, a restraining damping layer, a bearing layer and a sound absorbing layer. The constrained damping layer (1) and the load bearing layer (3) are respectively fixed on the top end and the bottom end of the support frame (4); The sound absorbing layer (2) is arranged between the constraint damping layer (1) and the bearing layer (3). The sound insulation device provided by the invention realizes good acoustic sealing to the transformer, and improves the sound insulation and noise reduction effect of the sound insulation cover at the position of the inlet and outlet lines. After the sound insulation device is applied, the noise value in the 2m area near the inlet and outlet lines is reduced by about 10dB (A) at most.