The invention discloses an ultrathin g-C3N4/MOF hybrid membrane and a preparation method thereof. The preparation method comprises the following steps: (1) preparing g-C3N4 powder; (2) preparing a g-C3N4 two-dimensional nanosheet solution; (3) preparing a g-C3N4/metal ion compound solution; and (4) sequentially depositing the g-C3N4/metal ion compound solution and the prepared organic ligand solution on a porous carrier through a spin coating technology, and growing into the ultrathin g-C3N4/MOF hybrid membrane loaded on the porous carrier. The method for the g-C3N4/MOF is applicable to preparation of multiple hybrid membranes and has wide application prospects. Meanwhile, the operation is simple and feasible, any toxic or harmful solvent is not used, secondary pollution is avoided, and large-scale industrial production can be realized.