The green and controllable synthesis of Au-Ag alloy nanoparticles (NPs) in one-pot has been a challenge in nanotechnology. Here, the soluble starch was employed as a reducing and capping agent to synthesize stable Au-Ag alloy NPs without using any toxic agent. The surface plasmon resonance wavelength and the composition of Au-Ag alloy NPs could be continuously adjusted only by the synthesis time in one-pot. Au-Ag alloy NPs were characterized by means of various techniques. It could be found as-prepared NPs were quasi-spherical with a small average size and a face-centered cubic polycrystalline structure. The above green synthesis method, facile, efficient and eco-friendly, can be extended to the controllable synthesis of other bimetallic NPs. Obtained Au-Ag alloy NPs exhibited a superior photocatalytic activity and stability for the 4-nitrophenol (4-NP) degradation due to abundance hydroxyl groups of the soluble starch and the synergistic effect between Au and Ag elements. The rate constant of 4-NP degradation could be linearly tuned by the composition of Au- Ag alloy NPs and their synthesis time besides their addition amount. The above methods to control the rate constant provide promising routes for other photocatalytic reactions using bimetallic NPs as photocatalysts.