The invention provides a method for directly growing an ultrathin porous graphene separation membrane and belongs to the technical field of membranes. An etching agent, an organic solvent and a high-molecular polymer are applied to metal foil and calcined at the high temperature under an oxygen-free condition; a metal substrate and a reaction product are removed, and the one-layer or multi-layer porous graphene separation membrane can be obtained. Alternatively, a solution or a dispersing agent of the etching agent is applied to metal foil, then the foil is covered with one layer of organic high-molecular polymer film and calcined at the high temperature under an oxygen-free condition, a metal substrate and a reaction product are removed, and the one-layer or multi-layer porous graphene separation membrane can be obtained. The method is simple, does not require expensive equipment and chemicals and is low in cost; the porous graphene separation membrane can grow out directly, and a graphene raw material is not required to be prepared in advance; the pore diameter of the prepared graphene membrane is adjustable, and the membrane has superhigh water flux and irreversible pollution resistance.