Small organic photothermal agents (PTAs) with dual photothermal and imaging functions in the second near-infrared (NIR-II) window present a promising strategy for deep tumor treatment, however, fluorescence quenching conventional PTAs and low photothermal conversion efficiency (PCE) present obstacles to their widespread application. In this study, a novel "dendritic donor engineering" strategy was employed to design NIR-II organic PTAs (named DCTBBT and TCTBBT) with donor-π-acceptor-π-donor features and aggregation-induced emission (AIE) activity. Owing to the fine-tuning of the dendritic donors, the close co-facial packing of the central π-backbone was disrupted, effectively avoiding fluorescence quenching caused by π-π aggregation, which facilitated molecule-free motions in aggregate state, and as a result, the DCTBBT nanoparticles (NPs) demonstrated a PCE of 59.8 %. Besides, both in vitro and in vivo evaluations demonstrate that DCTBBT NPs exhibit superior antitumor efficacy by the photothermal therapy (PTT). This study provides valuable insights into the development of advanced NIR-II PTAs for practical applications in phototheranostics.