Thin Pd-Ag alloy composite membranes were simultaneously deposited on commercial asymmetric porous stainless steel (APSS) tubes, using a combined method of electroless plating and electroplating under mild conditions. The X-ray diffraction patterns proved that the Pd-Ag alloy structure was formed at room temperature. The permeation behavior was investigated in detail, revealing that the hydrogen permeation flux, which was as high as 0.28 mol/(m(2) s), and the infinite hydrogen selectivity versus argon were achieved at a temperature of 773 K with a pressure difference of 100 kPa. The good membrane stability was proven by the temperature-changing cycles (11) and the gas-exchanging cycles (20). Electron probe microanalysis indicated that the Pd-Ag alloy composite membranes had a stable structure and a homogeneous Pd-Ag alloy layer. In addition, the membrane reactor for methane steam reforming was constructed and the reaction performance was tested. The methane conversion, which was as high as 80.72%, was achieved at a lower temperature of 773 K and a pressure of 500 kPa.