The development of nanoplatforms with integrated therapeutic and imaging functions is necessary for highly efficient cancer therapy. Herein, 3D CuS hollow nanoflowers (HNs) consisting of 2D nanoplates are successfully fabricated through the technique of laser ablation in liquids followed by ion-exchange reactions and applied for the first time as a theranostic nanoagent for magnetic resonance imaging (MRI), photothermal therapy (PTT), and chemotherapy simultaneously. Due to the sufficient and immediate contact between the exposed cupric centers of nanoplates and protons from water molecules, CuS HNs are demonstrated to be capable of being a T1 positive contrast agent for efficient MRI of tumors on the T2-weighted fluid-attenuated inversion recovery imaging (T2-FLAIR) sequence. Besides, the hollow structure makes CuS HNs an efficient nanoplatform for drug loading with a laser-triggered drug release. Moreover, CuS HNs exhibit high photothermal conversion efficiency (30%) and good biocompatibility. The combination of PTT and chemotherapy with CuS HNs provides a significant synergistic therapeutic effect, resulting in a higher tumor inhibition ratio than PTT or chemotherapy alone. This study demonstrates a single-component multifunctional theranostic nanoagent for T2-FLAIR MRI guided thermochemotherapy, which has great potential application in theranostics of cancer.