Circular RNAs (circRNAs), which are a class of non-coding RNA with covalently closed loops, play important roles in epigenetics regulation of gene expression at both the transcriptional and post-transcriptional level. Accumulating evidence demonstrated that numerous circRNAs were abnormally expressed in tumors and their dysregulation was involved in the tumorigenesis and metastasis of cancer. Although the functional mechanisms of many circRNAs have been revealed, why circRNAs are dysregulated in cancer remains elusive. CircRNAs are generated by a “backsplicing” process, which is regulated by different cis-regulatory elements and trans-acting proteins. Therefore, how these cis- and trans-elements change during tumorigenesis and how they regulate the biogenesis of circRNAs in cancer are two questions that interest us. In this review, we summarized the pathways for the biogenesis of circRNAs; and then illustrated why circRNAs dysregulated in cancer by discussing the changes of cis-regulatory elements and trans-acting proteins that related to circRNA splicing and maturation in cancer.