Background: Familial adenomatous polyposis (FAP) is a hereditary colorectal cancer (CRC). We performed genetic testing on nine FAP patients and identified a recurrent mutation at the 671st site of the MUC20 gene-MUC20-S671C. This mutation has a detection frequency of zero in the 1000 Genomes Project database. Previous studies have demonstrated that MUC20 can promote CRC progression through epithelial-mesenchymal transition (EMT). We conducted a series of experiments to analyze the impact of this mutation on CRC cells, aiming to infer its potential role and significance in CRC patients. Methods: We introduced the MUC20-S671C mutation into the CRC SW480 cell line using the CRISPR-Cas9 technique and established a stable cell line carrying this mutation. We then conducted various experiments to assess the effects of this mutation. The Transwell assay was used to evaluate cell invasion and migration. We also examined cell proliferation, cell cycle progression, and apoptosis rate. Furthermore, we tested the tumorigenic ability of these cells in NOD-scid IL2Rγ[null] (NSG) mice. Additionally, transcriptome sequencing was performed on both cell lines and mouse tumor tissues to obtain molecular regulatory network data, and key molecules were further validated. Results: The results of Cell Counting Kit-8 (CCK-8), 5-ethynyl-2'-deoxyuridine (EdU), and colony formation assays indicated that the proliferation ability of mutant cells was significantly reduced. The Transwell assay demonstrated a marked decline in the invasion and migration capabilities of mutant cells. Flow cytometry analysis revealed that the mutation increased the apoptosis rate of CRC cells and might have caused S-phase arrest. The tumor formation assay in nude mice showed that the tumorigenic ability of mutant cells was weakened. Transcriptome sequencing of both the cells and tumor tissues suggested that the mutation altered the expression of apoptosis- and cell cycle-related molecules and also affected EMT. Further experiments confirmed that key molecules involved in the EMT process, such as E-cadherin, were upregulated, while Vimentin, MMP9, and MMP14 were significantly downregulated, indicating that the mutation weakened the EMT capability of CRC cells. Conclusions: We have identified a novel mutation, MUC20-S671C, in patients with FAP. Our study demonstrates that this mutation exerts its tumor-suppressive effect by reversing the EMT process.