I-VI chalcogenide low-toxicity semiconductors and their near-infrared optical property are of great importance for solar cell and biological probe applications. Here, we report the synthesis of AgS Se (x = 0-1) ternary nanocrystals (NCs) and their photoelectrochemical properties, using a refined simple hot-injection reaction recipe. The ICP-MS results show the change of non-metallic composition in products and precursors, which can be well fitted with Vegard's equation. Ternary alloying broadens the absorption spectrum region of AgS NCs. It can also balance the transfer of photo-excited electrons through the interfaces of TiO/AgS Se and AgS Se /electrolyte by minimizing electron-hole recombination. By tuning the compositions, an increase in power conversion efficiency (PCE) was observed with the increase of S composition and the size of the NCs. The photoelectrochemical results reveal that AgS Se ternary NCs exhibit higher conversion efficiency than pure binary NCs. The drop in PCE of the binary NCs is mainly attributed to the decreases of the charge separation following exciton transition.