Exploring novel photosensitizer (PS) with good stability and high light converting efficiency and designing novel structure to integrate deep penetrating near-infrared (NIR) light excitable up-conversion nanoparticles (UCNPs) and PS into one system are highly fascinating in the photodynamic therapy (PDT) field. In this study, a novel core-shell structured platform (UCNPs@g-CN-PEG) with all-in-one "smart" functions for simultaneous photodynamic therapy, photothermal therapy (PTT), and trimodal imaging properties has been rationally designed and fabricated. This system is composed of a core-shell-shell structured NaGdF:Yb/Tm@NaGdF:Yb@NaNdF:Yb up-conversion luminescence (UCL) core and photoactive graphitic-phase carbon nitride (g-CN) mesoporous shell closely coated on individual core. This designed structure allows large specific surface area, high loading amount, close proximity to the UCL core, and almost no leakage of g-CN PS, thus ensuring sufficient reactive oxygen species (ROS) to damage tumor cells. Excitation by 808 nm NIR light, the emitted ultraviolet, and visible light can activate g-CN to generate significant amount of ROS and the doped Nd ions give rise to obvious thermal effect, which leads to excellent antitumor efficiency due to the combined PDT and PTT effect. Considering the trimodal imaging properties (UCL, computed tomography, and magnetic resonance imaging), we achieved an imaging guided cancer phototherapy motivated by a single NIR laser.