Rational manipulation of nonradiative decay channels is of crucial significance to improve photothermal conversion efficiency (PCE) and design photothermal agents. We first used the “internal and external combined” nonradiative decay strategy to enhance PCE. Specifically, organic IR-Y6 NPs with strong NIR absorption and high molar extinction coefficient were prepared and characterized. By means of TD-DFT calculations and fs-TA spectroscopy, the dual nonradiative decay channels composed of the free rotor (external strategy) and ultrafast dark excited states (DESs) between S0 and S1 states (internal strategy) were proved, which significantly enhanced PCE, up to 66%. IR-Y6 NPs were applied to a mice tumor model for photoacoustic image-guided photothermal therapy, showing complete tumor ablation ability and good biocompatibility for the normal organs. This work is of significance to deeply understand the nonradiation decay mechanism and rational design of high-performance PTT agents.