The invention provides a forming method and a forming device for an asymmetric membrane, in particular to an innovative design to better the filtration quality; wherein, the forming method is characterized in that a frame is used to conduct a series of treatments on the filterable membrane, namely, obtaining the thin membrane, coating and positioning, and cold and hot treatment, then the filter holes on a first and a second contact surfaces are respectively shaped into large and small apertures asymmetric at both ends, thus obtaining the finished membrane. By adopting the innovative design, the filter membrane in the invention has the advantages of simplifying the manufacturing process, lowering material costs, as well as improving the filtration quality of the filter membrane, and facilitating the cleaning. Furthermore, the membrane can be applied for filtering water or used in the technical field of medical treatment, so as to achieve the practicality and progress of simplifying the process and saving materials.