Metal additive manufacturing (AM) achieves a near-net-shaped forming capability and thus offers distinct advantages over conventional machining in fabricating components with complex internal channel geometries. Nevertheless, the as-fabricated channels typically exhibit suboptimal surface quality characterized by elevated roughness, partially sintered powder adhesion, and spherical melt droplets, all of which detrimentally affect component performance and operational reliability. The precision finishing of these intricate internal surfaces remains a significant technological bottleneck that hinders the broader industrial implementation of metal AM. This review comprehensively examines four principal surface enhancement techniques for internal channels: abrasive flow polishing (AFP), chemical polishing (CP), electrochemical polishing (ECP), and electrolyte plasma polishing (EPP). Through a systematic comparison of their process mechanisms, operational constraints, and recent technological advancements, we elucidate their respective merits and limitations and identify promising development directions. The analysis in this review is beneficial for providing crucial insights for surface quality optimization, addressing current AM challenges, and facilitating the industrial maturation of technology.