The liquid-liquid phase-transfer-catalyzed (PTC) Wittig reaction is a green and sustainable method to access alkenes from ketones or aldehydes, which consists of immiscible two liquid phases with reactions in both phases. Its conventional batchwise synthesis in stirred reactors is severely limited by a low mass transfer rate resulting from poorly defined liquid-liquid interfacial area and drop size control difficulties. In this work, this biphasic reaction in microchannels under the slug-flow pattern was experimentally and numerically studied. The influences of channel size and a range of operating parameters on the specific interfacial area, extraction efficiency, volumetric mass transfer coefficient, and conversion of this biphasic reaction at various operating conditions were thoroughly investigated. The results revealed that the specific interfacial area, extraction efficiency, and volumetric mass transfer coefficient decreased with channel size. For a fixed channel length, the extraction efficiency decreased with mixture flow velocity. When the residence time was kept constant, the volumetric mass transfer coefficient increased as the mixture flow velocity increased. The conversion increased with decreasing size of the channel, and it augmented with mixture flow velocity and aqueous-to-organic phase flow ratio as well. A CFD model coupling the two-way mass transfer and the two reactions was developed and well validated against the relevant experimental data, which enabled a better understanding of the spatiotemporal variation mechanism of this biphasic reaction on the microscale. The results presented would be helpful for the developments of industrial applications of microchannel-based PTC Wittig reactions with improved performance.