In recent years, great progress has been made for organic solar cells (OSCs), in which the innovation of active layer materials has played the critical role. Among different type photovoltaic materials, acceptor-donor-acceptor (A-D-A) structured oligomer-like small molecules, which possess the advantages of defined chemical structures, easily tunable energy levels and absorptions, and unique distribution of electron cloud, have received great attention in the OSC community. Significant progresses have been made, in which the A-D-A type materials including electron donor and electron acceptor materials play important roles. In this review, we start with the discussion of two representative A-D-A type oligomers like donors based on oligothiophene and benzodithiophene unit. OSCs with PCEs over 10% based on these electron donor materials have been realized with delicate molecular modifications and device optimizations. The relationship between the chemical structures and molecular properties is given. For the A-D-A type molecules, the electron-deficient ending groups (A) impose a clear impact on the lowest unoccupied molecular orbital (LUMO) energy levels, solubility, and the molecular packing. The conjugation length of the electron-donating central units (D) affects the highest occupied molecular orbital (HOMO) energy levels and molecular packing. Besides, side chains attached on the backbone can be used to fine-tune the molecular energy levels as well as molecular packings. All these factors lead to different optical and electrical properties as well as the charge transport properties, and thus the device performances. Then, two series of A-D-A type acceptors based on the central core, fluorene and BDT, have been reviewed. Different strategies are adopted to design and synthesize novel low bandgap acceptors. First, fused ring core based on the fluorene and BDT units are used to enhance and electron-donating properties, which help to up-shift the HOMO levels. Second, ending groups with strong electron withdrawing abilities are introduced to down-shift the LUMO levels. Third, the above two methods are combined to tune the HOMO and LUMO levels simultaneously. With those, novel low bandgap acceptors based on fluorene and BDT with PCE over 15% are demonstrated. Next, recent work on tandem OSCs in our group have been summarized. With continuous innovations of A-D-A materials and device optimizations, a record PCE of 17.36% has been obtained with the guide of semi-empirical model. Last, we propose the future and challenge of OSCs from the perspective of power conversion efficiency, device stability, flexible and large-area device.