Due to the unique properties of organic semiconductors, donor/acceptor materials in organic solar cells (OSCs) have to achieve nano-scale bicontinuous interpenetrating networks with a thickness of hundreds of nanometers to ensure efficient exciton dissociation and charge carrier transport simultaneously, thus realizing high power conversion efficiencies (PCEs). However, generally, it is very difficult to acquire optimal active-layer morphology only by using a pristine processing solvent, due to the distinct differences in the intrinsic properties of the donor and acceptor materials. It has been widely accepted that an additive-assisted strategy is an efficient and facile method to optimize the active-layer morphology and plays a crucial role in achieving high-efficiency OSCs. Recently, PCEs of OSCs have been significantly enhanced to over 19%, due to the rapid development of active-layer materials. Concomitantly, numerous novel additives are developed to meet the requirements of new materials and corresponding working mechanisms are also put forward, thus contributing to boosting PCEs. Herein, we summarize the recent advances of an additive-assisted strategy in OSCs with solvent additives, solid additives, and binary additives and their working mechanisms. Finally, we further outline the possible development directions after deeply understanding the status quo of the additive-assisted strategy in OSCs.