Two-dimensional (2D) metal-organic framework (MOF) nanosheet membranes demonstrate superhigh separation properties and offer great potential in practical separation applications, but the preparation of such membranes, especially tubular ones, still remains a huge challenge. Here, a vapor phase transformation strategy is presented for the direct synthesis of a 2D Co-based nanosheet MOF membrane on a porous tubular substrate via ligand vapor phase transformation of the Co-based gel layer coated on the substrate. The gel layer acts as a multifunctional role such as providing active metal source, guiding the oriented growth of the nanosheets and controlling the membrane thickness in the formation of the nanosheet membrane. The achieved Co-based nanosheet membrane with a thickness of around 57 nm exhibits excellent gas separation performance with H-2/CO2 ideal selectivity of as high as 58.7. This method shows great promise for the scalable fabrication of MOF nanosheet membranes.