A Z-scheme heterojunction with high separation efficiency of photogenerated electrons and holes and enhanced reduction/oxidation potentials, which can enhance reactive oxygen species generation and photothermal conversion efficiency, exhibits tremendous potential in photonic theranostics. Herein, antimonene nanosheets (Sb NSs) are functionalized with photosensitizer 5,10,15,20-Tetrakis(4-hydroxy-phenyl)-21H,12H-porphine (THPP) and a poly(ethylene glycol) (PEG) modifier. The Sb-THPP-PEG NSs thus fabricated are found to form a Z-scheme heterojunction structure between Sb and THPP, based on their valence band and bandgap level analysis. The Z-scheme heterojunction structure enables the Sb-THPP-PEG NSs multiple promising features for cancer therapy. Firstly, due to improved electron-hole pairs separation efficiency and redox potential, new reactive oxygen species center dot O-2(-) is generated, besides the production of O-1(2) by THPP. Secondly, the assembly of THPP enhances the NIR-light-to-heat conversion of Sb NS, a photothermal conversion efficiency as high as 44.6% is obtained by this Sb-THPP-PEG NSs photonic nanomedicine. Moreover, the photothermal, fluorescent, and photoacoustic imaging properties of Sb-THPP-PEG NSs allow multimodal imaging-guided tumor treatment.