Metasurfaces have shown unusual abilities to modulate the phase, amplitude and polarization of an incident lightwave with spatial resolution at the subwavelength scale. Here, we experimentally demonstrate a dielectric metasurface enabled with both geometric phase and magnetic resonance that scatters an incident light beam filling the full reflective 2π-space with high-uniformity. Specifically, by delicately reconfiguring the orientations of dielectric nanobricks acting as nano-half-waveplates in a metasurface, the optical power of phase-modulated output light is almost equally allocated to all diffraction orders filling the full reflection space. The measured beam non-uniformity in the full hemispheric space, defined as the relative standard deviation (RSD) of all scattered optical power, is only around 0.25. More interestingly, since the target intensity distribution in a uniform design is rotationally centrosymmetric, the diffraction results are identical under arbitrary polarization states, e.g., circularly polarized, linearly polarized or even unpolarized light, which brings great convenience in practical applications. The proposed uniform-backscattering metasurface enjoys the advantages including polarization insensitivity, high-integration-density and high-stability, which has great potential in sensing, lighting, laser ranging, free-space optical communication and so on.