Cysteine dioxygenase 1 (CDO1) gene is a non-heme structured, iron-containing metalloenzyme involved in the conversion of cysteine to cysteine sulfinic acid to regulate cysteine accumulation in vivo. Elevated levels of cysteine have been shown to be cytotoxic and neurotoxic, and this is the first important step in the breakdown of cysteine metabolism in mammalian tissues. The human CDO1 gene is located on chromosome 5q23.2. Studies have shown that deletion or epigenetic silencing of this chromosomal region contributes to tumorigenesis. It is highly expressed in the liver and placenta, and weakly in the heart, brain and pancreas. CDO1 is a tumor suppressor gene (TSG) with a wide range of functions, which can be involved in various biological processes such as tumor cell proliferation, differentiation, apoptosis and iron death, thus affecting the tumor development. CDO1 is epigenetically regulated in human cancers, compared to normal tissues. The CDO1’s mRNA or protein expression levels were significantly down-regulated in tumor tissues, whereas promoter DNA methylation of the CDO1 gene usually accumulates with the progression of human cancers. Aberrant hypermethylation on the CDO1 promoter is a common event in tumor cells, which leads to transcriptional inactivation and silencing of the CDO1 gene. High frequency of methylation of CDO1 gene promoter methylation region in a variety of tumors including breast, oesophageal, lung, bladder, gastric and colorectal cancers. CDO1 gene promoter methylation levels reflect cancer progression and malignant tumorigenesis, which is a common molecular indicator explaining poor prognosis in human cancers. Treatment with 5-aza-2′-deoxycytidine (a drug that promotes demethylation) reactivated the CDO1 expression in most cancer cell lines, indicating that the transcriptional expression of CDO1 is closely correlated with its promoter methylation level, CDO1 gene promoter methylation and tumor progression have also received increasing attention from researchers. It was found that CDO1 gene promoter hypermethylation can be used as an early tumor marker for clinical aid diagnosis and helps to differentiate cancerous from benign diseases. It was also found that CDO1 promoter DNA methylation showed reliable tumor monitoring potential in human body fluids, and furthermore, the degree of CDO1 promoter methylation was strongly correlated with resistance to chemotherapy with tumor drugs, which would be helpful in evaluating the efficacy of chemotherapeutic drugs. Thus, CDO1, a common promoter methylation gene in human cancers, is closely associated with the development of a wide range of tumors and is one of the most promising candidate genes for assessing tumor-specific epigenetic changes. This article reviews the biological functions of CDO1 and its promoter DNA methylation in tumors, focusing on the mechanism of CDO1 DNA promoter methylation in tumors, with a view to providing theoretical guidance for the clinical diagnosis and treatment of tumors with CDO1 as a potential therapeutic target.