Tumor-associated macrophages (TAMs), one of the dominating constituents of tumor microenvironment, are important contributors to cancer progression and treatment resistance. Therefore, regulation of TAMs polarization from M2 phenotype towards M1 phenotype has emerged as a new strategy for tumor immunotherapy. Herein, we successfully initiated antitumor immunotherapy by inhibiting TAMs M2 polarization via autophagy intervention with polyethylene glycol-conjugated gold nanoparticles (PEG-AuNPs). PEG-AuNPs suppressed TAMs M2 polarization in both in vitro and in vivo models, elicited antitumor immunotherapy and inhibited subcutaneous tumor growth in mice. As demonstrated by the mRFP-GFP-LC3 assay and analyzing the autophagy-related proteins (LC3, beclin1 and P62), PEG-AuNPs induced autophagic flux inhibition in TAMs, which is attributed to the PEG-AuNPs induced lysosome alkalization and membrane permeabilization. Besides, TAMs were prone to polarize towards M2 phenotype following autophagy activation, whereas inhibition of autophagic flux could reduce the M2 polarization of TAMs. Our results revealed a mechanism underlying PEG-AuNPs induced antitumor immunotherapy, where PEG-AuNPs reduce TAMs M2 polarization via induction of lysosome dysfunction and autophagic flux inhibition. This study elucidated the biological effects of nanomaterials on TAMs polarization and provided insight into harnessing the intrinsic immunomodulation capacity of nanomaterials for effective cancer treatment.