The present invention belongs to the field of medicine technology, particularly relating to an application of cyclic dinucleotide (cGAMP) in tumor treatment. Researches carried out in the present invention show that, cGAMP can inhibit growth of many types of tumor cells, with remarkable anti-tumor effect, thus, it can be used in preparation of anti-tumor drugs; and the prepared anti-tumor drugs have low toxicity and favorbale effect. Proved by a subcutaneousl tumor model in nude mice, cGAMP has a remarkable inhibition effect on tumors of humangastric carcinoma cell line MNK-45, human lung adenocarcinoma cell line A549, humancolorectal carcinoma cell line Lovo, human hepatocellular carcinoma cell line SMMC-7721, human prostaticcarcinoma cell line PC-3 and human pancreatic carcinoma cell SW1990, which are subcutaneously implanted in nude mice, and also proved by animal acute toxicity experiment. cGAMP has a relatively low acute toxicity, therefore and may be be used for preparing anti-tumor drugs.