Familial adenomatous polyposis (FAP) is a rare autosomal dominant genetic disease related to germline mutations of the APC gene. The clinical features of this disease most commonly include hundreds of adenomas or polyps. If not treated in a timely fashion, FAP can eventually result in colorectal carcinoma. In this report, clinical manifestations, family history, relevant auxiliary examinations and gene detection from patient blood led us to discover a novel frameshift mutation in exon 12 of the APC gene. The deletion of adenine in c.1439 resulted in the formation of codon 480. The occurrence of this frameshift deletion may lead to inexpressibility of the main functional regions in APC and may affect gene function. In addition, colonoscopy and histopathology showed malignant changes in the colon and rectum. There have been no reports of this frameshift mutation, but it can be considered in case of APC mutations and FAP in patients with clinical manifestations; auxiliary examination may be related, and it may be used as a reference for preventive clinical treatment in the future.