A series of composite membranes of poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) (P123) and polyacrylamide (PAM) have been successfully prepared by solvent-evaporation-induced self-assembly. Micellar self-assembly of P123 in aqueous solution plays an important role as a model for the formation of composite membranes. XRD patterns show that the synthesized compositions are in a lamellar mesostructure. The lattice spacing changes with P123 concentration: the higher the concentration of P123, the smaller the lattice spacing of the composite membranes. The data on sizes and zeta potentials of pure p-PAM aggregates, P123 micelles, p-PAM/P123 mixtures, and c-PAM/P123 composite aggregates suggest that interactions take place between PAM and P123 aggregations. The fabrication of the lamellar membranes via water-evaporation-induced self-assembly is recorded by fluorescent emission spectroscopy and dynamic light-scattering methods. Based on analysis of the results, a tentative mechanism for the formation of the lamellar membranes has been proposed.