The invention provides a method for testing the noise-reducing performance of a sound-insulating cover. The method comprises the following steps: a determining step, namely determining an initial sound pressure level of each frequency of an artificial sound source; an artificial sound source placing step, namely determining the position of the artificial sound source in the sound-insulating cover,wherein the sound-insulating cover is arranged outside an electric reactor; a probe placing step, namely determining the positions of multiple probes outside the sound-insulating cover; a first testing step, namely starting the artificial sound source, and receiving and recording a first sound pressure level of each frequency by each probe; a second testing step, namely removing the sound-insulating cover and the electric reactor, then starting the artificial sound source again, and receiving and recording a second sound pressure level of each frequency by each probe; and an analyzing step, namely analyzing the noise-reducing performance of the sound-insulating cover according to the difference between the first sound pressure level and the second sound pressure level corresponding to each frequency. According to the invention, the method for testing the sound-insulating cover is unified, the testing accuracy is effectively improved, the noise-reducing effect of the sound-insulating cover before and after mounting can be intuitively analyzed, and structural improvement and technological improvement of the sound-insulating cover are facilitated.