Objective: The use of immune checkpoint inhibitors (ICIs) provides promising strategies for hepatocellular carcinoma (HCC) treatment. This study aimed to explore impact and underlying mechanism of the combination therapy of quercetin and anti-programmed cell death 1 (anti-PD-1) antibody on HCC. Methods: Orthotopically transplanted HCC tumors in mice were treated with quercetin, anti-PD-1 antibody, or a combination of both therapies. Histopathological changes and programmed cell death ligand 1 (PD-L1) expression were characterized by hematoxylin and eosin (H&E) and immunohistochemistry (IHC) staining. The diversity and differences of gut microbiota (GM) were evaluated through 16S rRNA sequencing. Levels of macrophage immunity-related cytokines were quantified by enzyme-linked immunosorbent assay (ELISA), quantitative real-time polymerase chain reaction (RT-qPCR), and Western blot. Results: Combination therapy reduced necrosis, fibrosis, and PD-L1 expression in liver tissues. Additionally, combination therapy reduced GM imbalance and increased abundance of Firmicutes, Actinobacteria, and Verrucomicrobiota at the phylum level as well as Dubosiella and Akkermansia at the genus level. Combination therapy improved macrophage immunity, raised the expressions of CD8a, CD4, CD11b, interleukin (IL)-10, and interferon (IFN)-\gamma , and declined the expressions of IL-4, IL-6, toll-like receptor 4 (TLR4), an inhibitor of nuclear factor \kappaB\alpha (I\kappaB\alpha), and the NF\kappaB subunit p65. Upon combination therapy, expressions of M2 macrophage-related genes arginase-1 (Arg-1), IL-10, transforming growth factor-\beta (TGF-\beta), and matrix metalloproteinase-9 (MMP-9) were upregulated. Instead, M1 macrophage-related genes IL-6, IL-12a, IL-1\beta, and tumor necrosis factor-\alpha (TNF-\alpha) were downregulated. Conclusions: Quercetin/anti-PD-1 antibody combination therapy reshaped HCC tumor microenvironment in mice in parallel with regulating the GM and macrophage immunity.