The numbering-up of microchannel could cause negative effects on the hydrodynamics and mass transfer of gasliquid two-phase flow under inappropriate operating conditions. In this study, the hydrodynamics and mass transfer of gas-liquid two-phase flow in a tree-shaped parallel microchannel constructed by T-type bifurcations are investigated. Four flow regimes are obtained: bubbly flow, slug-foam flow, slug flow and compact slug flow. The results show that, there exists a critical ratio of gas flow rate to liquid flow rate to attain ideal flow distribution and optimal mass transfer performance. Beyond the critical point, the flow regime would become much different with before, the change of hydrodynamics could cause the remarkable deteriorations in flow distribution and mass transfer efficiency. The influencing mechanism of the operating conditions on hydrodynamics and mass transfer performance are analyzed. Furthermore, the correlations for predicting the volumetric mass transfer coefficient are proposed.