Using AMA nanosheets and Nafion as raw materials, several AMA-x@Nafion composite membranes were successfully synthesized by polymer solution casting and corresponding mechanical property, proton conductivity, water uptake and swelling, methanol permeability, and single-cell performance were investigated. It was found that AMA nanosheets were well dispersed in the matrix of the Nafion membrane due to strong electrostatic interactions among functional groups, including -SO3H and -NH2, overcoming the problem of poor dispersion of the composite membrane. Moreover, the content of AMA nanosheets had an important influence on the membrane performances, especially for methanol permeability. Among the prepared composite membranes, AMA-2.0@Nafion showed improved cell performance in contrast to recast Nafion. Notably, freezing water existed in the confined environment of the prepared composite membrane, which was significantly different from normal hydrocarbon-based PEMs. This provided a new idea and reference for the development of DMFCs used for a low-temperature cold startup.