Low bandgap conjugated polymers (LBG-CPs) are usually known as a class of conjugated polymers with energy bandgap below 1.5 eV. These polymers exhibit unique electronic and optical properties, particularly strong absorbance in the near-infrared (NIR) region. These unique characteristics make them highly desirable for use in various optoelectronic devices, including organic solar cells (OSCs), organic photodetectors (OPDs), organic thermoelectrics (OTEs), etc. Moreover, their capacity to convert light into fluorescence (FL), photoacoustic (PA), and thermal signals enables them as promising contenders for applications such as NIR fluorescence imaging (FLI), photoacoustic imaging (PAI), and photothermal therapy (PTT). In this chapter, we delve into the molecular structure design principles of LBG-CPs, recent advancements in the field, and their applications as light-harvesting materials, primarily focusing on OSCs, OPDs, OTEs, FLI, PAI, and PTT. By modulating the structure of LBG polymers, their optoelectronic properties, including absorption spectra, energy levels, and charge transport characteristics, can be rationally regulated by researchers. These efforts have led to the optimization of LBG-CPs, promoting the performance of OSCs and OPDs to unprecedented levels. Additionally, we address the current challenges and future prospects for enhancing the performance of LBG-CPs in various applications. By addressing these challenges and capitalizing on the potential of LBG-CPs, further advancements are speculated to be made in various fields. Overall, this chapter aims to provide insights into the molecular design, applications, and optimization of LBG-CPs as light-harvesting materials in some promising applications.