The present invention discloses a modified interleukin 12 (nsIL-12) and its gene, recombinant vector and use in manufacture of a medicament for treatment of tumors. When the oncolytic adenovirus vector carrying the modified interleukin 12 gene targets tumor tissue, the modified interleukin 12 is continuously expressed at a low level and mainly distributed in the local tumor tissue, which improves the specificity to tumor cells and reduces the systemic toxicity of interleukin 12; the modified interleukin 12 shows stronger inhibitory effect on tumor growth in intraperitoneally disseminated tumors and orthotopic tumors, and has low toxicity. The modified interleukin 12 armed oncolytic viruses show excellent anti-tumor effects, with a significant regression of tumors and lower toxicity compared with the existing IL-12 armed virus.