Background: Combination therapy is an effective method for augmenting the efficacy of immune checkpoint inhibitors (ICIs). Huaier is a commonly used Chinese patent medicine with substantial antitumor effects. The combination of Huaier and ICIs may increase the efficacy of ICIs against hepatocellular carcinoma (HCC). Methods: The major components of Huaier were detected by high-performance liquid chromatography-mass spectrometry. The optimal antitumor dose of Huaier was investigated in H22-bearing mice. Next, Huaier was combined with anti-CD8 alpha antibody (Ab) or anti-PD-L1 Ab to observe the antitumor effect. The safety of these combination drugs was evaluated through blood biochemical tests and hematoxylin and eosin staining of histological sections. RT-qPCR, immunohistochemistry, flow cytometry, and transcriptome sequencing were performed to investigate the potential action mechanism of anti-PD-L1 Ab combined with Huaier against HCC. Results: HPLC-MS/MS identified 333 components of Huaier, including carboxylic acids and derivatives, thienothiophenes, phenols, flavonoids and so on. Huaier exhibited significant antitumor effects, with the strongest effect noted at a dose of 4 g/kg. Huaier boosted CD8+ T cells infiltration into the tumor. Next, CD8+ T cells were depleted by with anti-CD8 alpha Ab, and the antitumor effect of Huaier was suppressed. Flow cytometry results revealed that CD8+ T cells were reduced in the Huaier+anti-CD8 alpha Ab group, with the antitumor effect of this group being inhibited. This indicated that CD8+ T cells were key players in the antitumor activity of Huaier. Meanwhile, Huaier inhibited microvessel density (MVD), downregulated vascular endothelial growth factor A (VEGFA), and upregulated PD-L1 in tumor tissues. Finally, Huaier combined with anti-PD-L1 Ab exhibited a greater antitumor effect in the H22-bearing mice. And the results of liver and kidney function tests and histological section analysis unveiled that the safety of these drugs was excellent. According to the transcriptome sequencing results, Huaier combined with anti-PD-L1 Ab possibly exerted anti-HCC effects through immunomodulation, immune response, and so on. Conclusions: Huaier exhibited a significant antitumor effect. It promoted CD8+ T cells infiltration, upregulated PD-L1 expression, downregulated VEGFA expression, and inhibited MVD, thereby playing a significant antitumor immunoregulatory effect. The combination of Huaier and anti-PD-L1 Ab has significant antitumor effects, and this regimen has good safety. Therefore, Huaier combined with anti-PD-L1 Ab is a promising therapeutic approach against HCC.