Traditional photothermal therapy requires high-intensity laser excitation for cancer treatments due to the low photothermal conversion efficiency (PCE) of photothermal agents (PTAs). PTAs with ultra-high PCEs can decrease the required excited light intensity, which allows safe and efficient therapy in deep tissues. In this work, a PTA is synthesized with high PCE of 88.3% based on a BODIPY scaffold, by introducing a -CF3 "barrier-free" rotor on the meso-position (tfm-BDP). In both the ground and excited state, the -CF3 moiety in tfm-BDP has no energy barrier to rotation, allowing it to efficiently dissipate absorbed (NIR) photons as heat. Importantly, the barrier-free rotation of -CF3 can be maintained after encapsulating tfm-BDP into polymeric nanoparticles (NPs). Thus, laser irradiation with safe intensity (0.3 W cm(-2), 808 nm) can lead to complete tumor ablation in tumor-bearing mice after intravenous injection of tfm-BDP NPs. This strategy of "barrier-free rotation" provides a new platform for future design of PTT agents for clinical cancer treatment.