Additive manufacturing (AM) technology enables a new way for fabricating components with complex internal surfaces. Selective laser melting (SLM), being one of the most common AM techniques, is able to fabricate complex geometries with superior material properties. However, due to the poor surface quality, the fabricated internal surfaces cannot meet the specifications for some real applications. To achieve the required internal surface condition, post-polishing process is essential. As one of the most prominent processes for finishing inaccessible surfaces with a wide range of materials, abrasive flow machining (AFM) shows great potential to polish AM internal surfaces. Hence, this paper presents an analytical and experimental study on the internal surface quality improvement of SLM Inconel 718 by AFM, aiming to verify the feasibility of AFM on internal surface quality improvement. The surface evolution process was modeled, and the effects of process parameters on surface and subsurface quality were evaluated. The results show that good surface roughness was obtained at the medium conditions of high viscosity, large particle size, low extrusion pressure, and low temperature. The surface morphology was greatly affected by the medium particle size which showed consistency with the surface evolution model that small abrasive particles are unable to overcome the width and depth of the valleys, resulting in the formation of craters. The partially melt layer was effectively removed, and no subsurface damage was induced.