Background: Given malignant ascites with a terrible prognosis and a unique immune microenvironment, our purpose is to evaluate whether oncolytic herpes simplex virus type 2(OH2) is able to safely eliminate ascites of colon cancer and through which specific mechanism it exerts antitumor immunity. Methods: We established an ascites mice model through intraperitoneal injection of CT26 cells and obtained an appropriate dose range for in vivo tests. Efficacy and safety of OH2 were detected by weight of ascites, blood routine analysis, histopathological examination, and the survival time of mice. The specific mechanism underlying antitumor immunity was analyzed by cytometric bead array, flow cytometry, and single-cell RNA sequencing. Furthermore, anti-interleukin (IL)-6R antibody tocilizumab was synchronously or sequentially delivered with OH2 to explore the role of the regional cytokine storm, mainly IL-6 hypersecretion. Results: OH2 was able to eliminate ascites and significantly prolong the survival of mice-bearing CT26 tumor cells by intraperitoneal injection, without obvious systemic damage to the main organs even though a regional cytokine storm. Hypersecretion of pro-inflammatory cytokines, mainly IL-6, and increased infiltration of CD4+ and CD8+ T cells were observed in ascites mice treated by OH2, compared with those treated by 5-fluorouracil or nonresponders. Furthermore, the initial-stage blocking of the IL-6 pathway was able to considerably suppress antitumor immune responses driven by OH2. Surprisingly, we discovered upregulations of the immune checkpoint genes such as Cd274 and Pdcd1 by single-cell RNA sequencing. Conclusions: OH2 could safely eliminate malignant ascites of colon cancer and convert the cold immune microenvironment by inducing a remarkably regional cytokine storm in ascites, mainly IL-6, in the early stage of antitumor immune responses beyond directed oncolytic virotherapy. Keywords: colon cancer; cytokine; herpes simplex virus type 2; malignant ascites.