The present invention provides a noise-reducing device for a transformer substation. A composite acoustic device comprises a bracket, a sound-absorbing body connected to the bracket, and a sound-insulating body pivotally connected to the bracket and parallel to the sound-absorbing body. According to the technical scheme of the invention, the low-frequency noise in the noise radiation is blocked and absorbed by the angle-adjustable sound-insulating body. Meanwhile, the low-and-medium-frequency noise is further absorbed by the sound-absorbing body. The sound-absorbing body and the sound-insulating body are used in combination to fully absorb the low-and-medium-frequency noise, so that the noise reduction effect is enhanced.