Near-infrared (NIR) absorbing nanoagents with functions of photoacoustic imaging (PAI) and photothermal therapy (PTT) have received great attention for cancer therapy. However, endowing them with multifunctions, especially targeting ability, for enhancing in vivo PAI/PTT generally suffers from the problems of synthetic complexity and low surface density of function groups. We herein report high density glycopolymers coated perylenediimide nanoparticles (PLAC-PDI NPs), self-assembled by poly(lactose)-modified perylenediimide (PLAC-PDI), as tumor-targeted PAI/PTT nanoagents. Atom transfer radical polymerization and click reaction were used in sequence to prepare PLAC PDI, which can accurately control the content of poly(lactose) (PLAC) in PLAC-PDI and endow PLAC PDI NPs with high density PLAC surface. The high density PLAC surface piovided NPs with long-time colloidal stability, outstanding stability in serum and light, and specific targeting ability to cancer cells and tumors. Meanwhile, PLAC-PDI NPs also presented high photothermal conversion efficiency of 42% by virtue of strong pi-pi interactions among perylenediimide molecules. In living mice, PAI experiments revealed that PLAC-PDI NPs exhibited effective targeting ability and enhanced PTT efficacy to HepG2 tumor compared with control groups, lactose blocking, and ASGP-R negative tumor groups. Overall, our work provids new insights for designing glycopolymers-based therapeutic nanoagents for efficient tumor imaging and antitumor therapy.