Abstract: Circular RNA (circRNA) is a novel non-coding RNA with unique characteristics. It is a closed circular molecule formed by a covalent bond without 5' end cap and 3' end tail. CircRNA has certain stability due to its resistance to exonuclease. It is widely expressed in mammals, and express differently in diverse tissues and development stages. CircRNA has become a research hotspot in recent years due to its special characteristics. At present, a large number of studies have found that circRNA is involved in cell senescence, proliferation, apoptosis, inflammation and other pathophysiological processes, and plays an important role in the occurrence and development of a variety of diseases. Of course, circRNA in tumor research is also starting. Many researches have confirmed that the expression levels of circRNA in tumor tissues and adjacent normal tissues are significantly different, and the relevant circRNA expression levels are correlated with the malignancy and invasiveness of tumor cells. Currently, most studies have confirmed that circRNA has the response element of miRNA, which can be used to adsorb the related miRNA like a sponge through the principle of complementary base pairing, so as to regulate the expression of downstream genes. Especially in various tumors, the mechanism of sponge-like action has been increasingly mature. In addition, circRNA can also be translated into protein or polypeptide to regulate gene expression. The mechanism of circRNA is complex and diverse, and its specific molecular mechanism has become the research direction of many scientific researchers. CircRNA is not only expressed specifically in tissues, but also in plasma and exosomes, and the expression levels are different, which further confirmed that circRNA can be used as a potential biomarker. In this paper, a large number of literatures were reviewed to briefly review the biological characteristics and mechanism of circRNA and its research progress and application value in various tumor diseases.