Ternary polymer solar cells (PSCs) have been widely demonstrated as an effective method to improve device performance.Meanwhile,the simple fabrication technology can be well kept with a single bulk heterojunction active layer,which is beneficial to the potential industrialization of PSCs.The photovoltaic parameters,such as short-circuit current density,open-circuit voltage,and fill factor,of ternary PSCs can be simultaneously improved by appropriately incorporating the third component,resulting in the increased power conversion efficiency (PCE) compared with the corresponding binary PSCs.To date,the PCE of ternary PSCs has exceeded 17% with the development of effective donors and non-fullerene acceptors.The fundamental designing rules of temary PSCs are summarized to give some available guidelines for material selection.Next,the recent progress of efficient ternary PSCs based on various types is investigated.Then,some methods are introduced to investigate the working mechanisms of temary PSCs.At length,the perspective toward the future development of ternary PSCs is discussed.