Nanotheranostic agents of gold nanomaterials in the second near-infrared (NIR-II) window have attracted significant attention in cancer management, owing to the reduced background signal and deeper penetration depth in tissues. However, it is still challenging to modulate the localized surface plasmon resonance (LSPR) of gold nanomaterials from the first near-infrared (NIR-I) to NIR-II region. Herein, a plasmonic modulation strategy of gold nanorods (GNRs) through manganese dioxide coating is developed for NIR-II photoacoustic/magnetic resonance (MR) duplex-imaging-guided NIR-II photothermal chemodynamic therapy. GNRs are coated with silica dioxide (SiO ) and then covered with magnesium dioxide (MnO ) to obtain the final product of GNR@SiO @MnO (denoted as GSM). The LSPR peak of GNRs could be tuned by adjusting the thickness of the MnO layer. Theoretical simulations reveal that this plasmonic modulation is mainly due to the change of refraction index around the GNRs after coating with the MnO layer. Additionally, the MnO layer is demonstrated to degrade into Mn ions in response to peroxide and acidic protons in the tumor microenvironment, which allows for MR imaging and chemodynamic therapy. This plasmonic modulation strategy can be adapted to other metal nanomaterials and the construction of a new class of NIR-II nanotheranostics.