Obesity usually induces systemic metabolic disturbances, including in the tumor microenvironment (TME). This is because adaptive metabolism related to obesity in the TME with a low level of prolyl hydroxylase-3 (PHD3) depletes the major fatty acid fuels of CD8 T cells and leads to the poor infiltration and unsatisfactory function of CD8 T cells. Herein, we discovered that obesity could aggravate the immunosuppressive TME and weaken CD8 T cell-mediated tumor cell killing. We have thus developed gene therapy to relieve the obesity-related TME to promote cancer immunotherapy. An efficient gene carrier was prepared by modifying polyethylenimine with -methylbenzenesulfonyl (abbreviated as PEI-Tos) together with hyaluronic acid (HA) shielding, achieving excellent gene transfection in tumors after intravenous administration. A/EI-Tos/NA () containing the plasmid encoding PHD3 (PHD3) can effectively upregulate the expression of PHD3 in tumor tissues, revising the immunosuppressive TME and significantly increasing the infiltration of CD8 T cells, thereby improving the responsiveness of immune checkpoint antibody-mediated immunotherapy. Efficient therapeutic efficacy was achieved using together with αPD-1 in colorectal tumor and melanoma-bearing obese mice. This work provides an effective strategy to improve immunotherapy of tumors in obese mice, which may provide a useful reference for the immunotherapy of obesity-related cancer in the clinic.