Acoustic metasurfaces are artificial 2D structures with a sub-wavelength thickness that can realize some exotic properties such as non-trivial refraction, broadband and low frequency absorption. However, most relevant studies are still in a static medium, hindering their realistic applications in aviation, where background flow exists. To address it, the effects of mean flow on the acoustic performance of metasurfaces, which is designed based on the generalized Snells law (GSL) to achieve anomalous reflections, are systemically studied. Firstly, an analytical model of GSL taking the effect of background uniform mean flow into account is built, in which the wavenumbers of both incident and reflected waves are corrected. Then, taking an acoustic porous metasurface for instance, the effectiveness of the derived model is validated by numerical simulations. Results reveal that the reflected waves are deflected in the presence of background flow. The critical incident angle, at which the incident sound wave is converted to surface wave, decreases with the increasing flow velocity. Since the converted surface wave can only propagate along the metasurface, there is little sound energy radiated into far field, which is benefit for the noise attenuation in the presence of flow.