One-pot synthesis of gold nanoparticles in aqueous phase is of special interest. We herein report a facile strategy, namely, flash nanoprecipitation (FNP), based on a multi-inlet vortex mixer for preparation of stable gold nanoparticles with a size <3 nm. A rational design of different synthesis factors including concentrations of gold salt, reduction agent, and capping ligand, as well as flashing speed, is established. Changing the terminal groups of the capping ligands hardly impacts the formation and size of gold nanoparticles, while it does introduce tunable properties, for example, a different catalytic performance on the reduction of 4-nitrophenol. Therefore, the reported FNP demonstrates not only a robust method for the synthesis of gold nanoparticles but also tunable functions from the capping ligands for a variety of potential applications.