Objective:To investigate the tumor perfusion enhancement induced by low intensity ultrasound stimulated microbubble cavitation (USMC) combined with programmed cell death-Ligand 1(PD-L1) antibody on improving the immune microenvironment of solid tumors.Methods:Tumor-bearing mice were divided into 4 groups: Control ( n=26) group, USMC ( n=27) group, anti-PD-L1 ( n=27) group and USMC+ anti-PD-L1 ( n=27) group. USMC treatment was performed with a VINNO 70 ultrasound theranostics system. Tumor perfusion was evaluated by contrast-enhanced ultrasound (CEUS). The anti-tumor efficacy was assessed by the tumor growth curve and the survival time of mice. The number and function of CD8 + T cells, the differentiation of CD4 + T cells, the proportion of MDSC and the phenotype distribution of TAM in tumors were analyzed by flow cytometry. The content of CXCL9, CXCL10 and HIF-1α in tumor were detected by ELISA. The expression of VEGF in tumor tissues was analyzed by immunofluorescence. Results:CEUS showed that the values of PI and AUC of tumors were significantly increased after USMC compared with before USMC (all P<0.05). USMC combined with anti-PD-L1 therapy did suppress the tumor progression. FCM showed the number, the expression of proliferation antigen Ki67, the secretion of IFN-γ and Granzyme B of CD8 + T cells in tumors were higher in combined group than those in other three groups after therapy (all P<0.05). Meantime, the proportion of Th1 was rose while Tregs and MDSC were declined and the polarization of TAM was toward M1 type by combined therapy. ELISA analysis showed that the combined therapy also increased the concentration of CXCL9, CXCL10 and decreased the content of HIF-1α in tumors (all P<0.05). Meanwhile, the immunofluorescence expression of VEGF was significantly lower in combined group than that in the control group after treatment ( P<0.05). Conclusions:Tumor perfusion enhancement by USMC combined with PD-L1 antibody therapy could improve tumor immune microenvironment and USMC might be a novel effective method for potentiating PD-L1 antibody immunotherapy.