Although the growth mechanisms (e.g., seed-induced growth and capping agent orientation) of bimetal nanocrystals (e.g., core–shell, alloy, segmented, and branched) from artificial experimental speculation and theoretical calculation have been widely accepted, precisely revealing their growth mechanisms is still tremendously challenging. In this work, we utilized redox reaction kinetics for the first time to successfully reveal the aqueous sequential growth mechanism between Au and Ag nanocrystals of segmented Ag-Au-Ag heterojunction nanorods (HJNRs) in a one-step and high-temperature aqueous system. Herein, electrode potentials of different electrical pairs (e.g., Ag+/Ag and AuCl4 –/Au) at 200 °C could be calculated through using the Helgeson–Kirkham–Flowers state and other equations, from which whether Au and Ag nanocrystals grew successively and formed segmented Ag-Au-Ag HJNRs could be correctly assessed. The redox reaction kinetics mechanism can also explain well the aqueous-phase growth mechanisms of other bimetal nanocrystals and paves a promising avenue for the design and synthesis of other one-dimensional segmented metal nanostructures.