Au-Ag gradient alloy nanoparticles were directly synthesized in a microreaction system with their surface plasmon resonance been facilely adjusted. The surface plasmon resonance wavelength was red-shifted through increasing the raw ratio of Au³⁺:Ag⁺, decreasing the synthesis temperature or the residence time. A linear relationship was found between the surface plasmon resonance wavelength and the synthesis temperature, or the residence time. The range of surface plasmon resonance wavelength of monodispersed Au-Ag gradient alloy could be extended to 548 nm generated on the enrichment of Au as outer layer. It provided a suitable way to prepare Au-Ag gradient alloy NPs with longer surface plasmon resonance wavelength than 520 nm (Au) at low temperature.