Surface wettability plays an important role in industrial processes and the natural world. With their great potential applications in filtration and distillation, membranes with controlled surface wettability have recently drawn a lot of attention. The function of membranes is closely linked to their fluid wetting behavior. However, traditional membranes exhibit homogeneous wetting behavior on both sides due to their symmetric surface wettability, thus limiting their performance in specific scenarios. For example, unidirectional fluid transport and selective liquid separation are hard to achieve with symmetric membranes alone. In recent years, Janus membranes have emerged as a novel class of functional membranes, offering solutions to address these challenges, thanks to their distinctive asymmetric surface wettability. As a result of the asymmetric characteristic, Janus membranes enable a wide range of distinctive fluid wetting features, such as unidirectional fluid transport and selective fluid separation. These unique fluid transport behavior relies on the cooperative effects stemming from the asymmetric wettability across the membranes. Currently, there are two primary methods for the fabrication of Janus membranes: single-layer modification and double-layer composite. The former involves asymmetric surface modification of a membrane to create a Janus feature. While this approach offers control over the specific wettability of membranes, it is usually limited by strict reaction types and conditions. On the other hand, the double-layer composite method concerns cementing two separately prepared membranes to produce an asymmetric membrane. Although this approach is simpler than the surface modification, mismatched cementation might arise, resulting in increase of transport resistance. In this context, precise creation and regulation of asymmetry remains a key challenge in the development of Janus membranes.