A metal tube-in-tube microchannel (MTMC) with a high throughput was developed based on a typical T-junction microchannel. In the MTMC, two coaxial stainless steel tubes formed an annular microchannel, and a micropore section on the annular surface of one end of the inner tube was employed as the dispersion medium. The physical absorption process of CO₂ in water in the MTMC was investigated. The effects of varying the gas-liquid contact method, superficial gas and liquid velocities, and structural parameters of the MTMC (including micropore size and annular size) on the liquid side volumetric mass transfer coefficient were explored. The experimental results indicated that the flow of gas in the inner tube and liquid in the outer tube was beneficial to mass transfer. The liquid side volumetric mass transfer coefficient increased with the increase of superficial gas and liquid velocities, with the superficial liquid velocity having a greater effect. In addition, decreasing the micropore size or annular size both increase the mass transfer coefficient; however, annular size has a more obvious effect.