A method for producing a protein polymer fiber, the method comprising providing a liquid protein solution in a container for liquid, and repeatedly moving the liquid surface in the container back and forth between a first and a second position. Said movement of the liquid surface is such that the protein polymer solution is allowed to form a film in the interface between the liquid surface of the liquid protein solution and a surrounding fluid. The movement of the liquid surface being performed by respectively raising and lowering the liquid surface relative to the container or by moving an object extending through the liquid surface of the liquid protein solution. Also, a device for performing said method.