Rotating bar reactor (RBR), a promising equipment for process intensification, has shown excellent micromixing efficiency. However, the mass transfer performance of the RBR, which is a crucial indicator for evaluating the application prospect, is still unclear. In this work, the mass transfer study of RBR was conducted using water-air physical absorption system. Different internals, including gas distributor (GD) for optimizing gas dispersion and liquid disturbing pin (LDP) for promoting liquid disturbance, were employed to enhance the mass transfer. In comparison with RBR without internal, the enhancement of mass transfer was about 38% at rotational speed of 1000 r/min. The gas flow behavior was observed via high-speed photography. In the presence of internals, the bubble size decreased and the distribution was more uniform. The rising velocity of bubbles decreased about 10%. The result indicated that introducing internals was conducive to improving gas-liquid interfacial area and residence time.