Objective To systematically review the relationship between obesity and the incidence of digestive system cancers. Methods The PubMed, EMbase, The Cochrane Library, CNKI and WanFang Data databases were electronically searched to collect cohort studies on the relationship between obesity and digestive system cancers from January 1st, 2001 to October 31st, 2021. Two reviewers independently screened the literature, extracted data, and assessed the risk of bias of the included studies. Meta-analysis was then performed by using RevMan 5.4 software. Results A total of 16 cohort studies were included. The results of meta-analysis revealed that compared with normal weight, obesity increased the incidence rate of various cancers of the digestive system, including colorectal cancer (RR=1.25, 95% CI 1.13 to 1.39, P<0.000 1), liver cancer (RR=1.65, 95%CI 1.41 to 1.92, P<0.000 01), pancreatic cancer (RR=1.34, 95%CI 1.19 to 1.51, P<0.000 01), gastric cancer (RR=1.09, 95%CI 1.05 to 1.14, P<0.000 1), and esophageal cancer (RR=2.39, 95%CI 1.98 to 2.89, P<0.000 01). Conclusion The current evidence indicates that obesity can increase the incidence rate of digestive system cancers. Due to the limited quality and quantity of the included studies, more high-quality studies are needed to verify the above conclusion.