Abstract: A magnesium-based fly ash porous sound-absorbing material with a surface hydrophobically modified, and a preparation method thereof are provided. In the preparation method, a basic magnesium sulfate cement is adopted as a cementing agent and a fly ash is adopted as a mineral admixture to prepare a slurry; foaming is conducted through a physical foaming process in a foaming machine to obtain a foam; and the foam is mixed with the slurry, and a resulting mixture is poured and cured, and then subjected to a surface hydrophobic modification through vapor deposition to obtain the sound-absorbing material. The sound-absorbing material has a density of 251 kg/m3 to 306 kg/m3, a noise reduction coefficient (NRC) of 0.65 to 0.7, a compressive strength of 1.8 MPa to 2.2 MPa, and a water contact angle of 129° to 151°.