Simple Summary: Despite significant advances using immune checkpoint blockade therapy for patients with advanced cancer, 40-85% of patients fail to respond to therapy. Increasing evidence suggests that the tumor immune microenvironment plays a critical role in rendering the immune checkpoint blockade ineffective. As such, targeting the tumor immune microenvironment is an attractive strategy for the treatment of solid tumors and an important approach to improve the efficacy of checkpoint inhibitors. Our study explores the combination therapy of DNase I and PD-1 antibody to prevent tumor growth. We demonstrate that this combination treatment for colorectal cancer shows superior efficacy in comparison to a single-agent treatment, both in vivo and in vitro.Immune checkpoint inhibitors can improve the prognosis of patients with advanced malignancy; however, only a small subset of advanced colorectal cancer patients in microsatellite-instability-high or mismatch-repair-deficient colorectal cancer can benefit from immunotherapy. Unfortunately, the mechanism behind this ineffectiveness is unclear. The tumor microenvironment plays a critical role in cancer immunity, and may contribute to the inhibition of immune checkpoint inhibitors and other novel immunotherapies in patients with advanced cancer. Herein, we demonstrate that the DNase I enzyme plays a pivotal role in the degradation of NETs, significantly dampening the resistance to anti-PD-1 blockade in a mouse colorectal cancer model by attenuating tumor growth. Remarkably, DNase I decreases tumor-associated neutrophils and the formation of MC38 tumor cell-induced neutrophil extracellular trap formation in vivo. Mechanistically, the inhibition of neutrophil extracellular traps with DNase I results in the reversal of anti-PD-1 blockade resistance through increasing CD8+ T cell infiltration and cytotoxicity. These findings signify a novel approach to targeting the tumor microenvironment using DNase I alone or in combination with immune checkpoint inhibitors.