Cancer has a high mortality and prevalence rate in the world. CRISPR-Cas9 is one of the novel and most common gene-editing techniques. Compared with the first two generations of gene-editing technologies, CRISPR-Cas9 system has the advantages of easy design, low cost, high efficiency and so on. sgRNA guides Cas9 to the site of the targeted gene, and Cas9 cuts the DNA strand at that site, triggering the NHEJ or HDR mechanism so as to achieve the purpose of deletion or insertion. CRISPR-Cas9 can be combined with other factors for other purposes, such as CRISPRa, CRISPRi, and base editing. The CRISPR system now has been used extensively for research into biological mechanisms and disease treatments. Since cancer is controlled by genes, a number of researchers in recent years have looked at using the CRISPR system to treat cancer. The CRISPR technology has greatly improved our understanding of cancer and the factors that affect it, and has had a major impact on the study and treatment of cancer. CRISPR gene editing can quickly and efficiently generate gene knockouts and regulate gene expression to identify relevant genes that influence cancer growth. This review systematically introduces CRISPR-Cas9 and its application methods, delivery modes, and discusses some studies using cell lines and organoids in vitro and animal models for cancer therapy in vivo.