Recently, Ag₂Se has attracted great interest due to its excellent room-temperature thermoelectric (TE) performance, interesting mechanical properties, and environmental friendliness. Tuning the Ag stoichiometry is a conventional strategy to optimize the TE performance of Ag₂Se-based materials, but the effects of Ag content on the mechanical properties are still unclear. In this work, we find Ag-excess has weak influence on the plasticity of Ag₂Se, but can significantly enhance the plasticity of S-alloyed Ag₂Se. The Ag-excessive Ag_2.01Se_0.7S_0.3 can endure large bending strain above 20% without cracking, comparable with those of many metals. The improved plasticity is possibly caused by the increased density of Ag—S bonds. The TE figure-of-merit of plastic Ag_2.01Se_0.7S_0.3 reaches 0.53 at room temperature. The flexible TE device consisting of free-standing Ag_2.01Se_0.7S_0.3 films shows high maximum material normalized power density up to 0.20 W/m under the temperature difference of 25 K. This work sheds light on the further investigation of mechanical properties of Ag-based TE materials.