Elevating photothermal conversion efficiency (PCE) is the key to enhance curative efficacy during photothermal therapy. Nevertheless, current functionalized or synthetic methods for preparing photothermal agents with high PCE are too complicated, greatly hindering the development of photothermal therapy. In this study, we developed a tumor-targeting flower-like nanocomposite termed mesoporous platinum nanoparticles@Cu2-xSe-AS1411 (MPNPs@Cu2-xSe-AS1411, denoted as MPCSA) with remarkable PCE for high-resolution photoacoustic (PA) imaging guided photothermal/chemodynamic therapy for tumor ablation. We discovered that MPNPs served as carriers for delivering PA contrast agents. Cu2-xSe nanosheets emerged as promising near-infrared-II (NIR-II) PA imaging and photothermal/chemodynamic therapy agents owing to their strong localized surface plasmon resonance property in the NIR-II region. The amine-modified AS1411 aptamer, which is connected to the surface of the flower-like MPNPs@Cu2-xSe can target the nucleolar protein on the surface of 4T1 cells, achieving the accumulation of MPCSA at the tumor site in mice. Notably, these MPCSA nano-flowers exhibited an excellent PCE up to 82.33 % under 1064 nm laser irradiation, resulting in highly efficient cancer cell ablation and greatly hindering in vivo tumor growth. This study has paved the way for imaging-guided accurate cancer diagnosis and targeted cancer therapy.