Circular RNAs (circRNAs) are endogenous non-coding RNAs that widely exist in eukaryotic transcriptomes. Unlike linear RNAs, circRNAs are not easily degraded for having a more stable structure. By acting as microRNA (miRNA) sponges, RNA-binding protein sponges, regulating transcription, or translating to proteins, circRNAs regulate gene expression at transcriptional or post-transcriptional levels. Recent studies have found that circRNAs play important roles in regulating cell proliferation, migration and invasion of tumor cells. Gastrointestinal tumors are common malignant tumors with high mortality in China. Researchers found a large number of abnormally expressed circRNAs (such as hsa_circ_0014717, hsa_circ_0001017, hsa_circ_0061276, hsa_circ_0125965, hsa_circ_0000181, hsa_circ_002059, hsa_circ_0000190, hsa_circ_0000096, hsa_circ_0014717, and hsa_circ_KLDHC10) in gastrointestinal tumors. This paper firstly describes the formation mechanisms and biological functions of circRNAs. Then, combining with the latest research progresses of gastrointestinal tumor-associated circRNAs and our group’s related research results, we review the mechanisms of circRNAs affecting gastrointestinal tumorigenesis and hope to provide a new theory for the diagnosis and treatment of gastrointestinal tumors.