Background: The incidence of colorectal cancer among persons aged < 50 years (early-onset colorectal cancer, EOCRC) has increased since the early 1990s. However, the risk factors contributing to this trend remain largely unknown. Methods: We conducted a retrospective study of participants who were aged < 50 years and without a previous cancer history, using the China Kadoorie Biobank cohort study. We analyzed data related to demographics, lifestyle habits, family history, and comorbidities of EOCRC cases with participants without colorectal cancer in this age group (controls). Univariate and multivariate-adjusted cox regression models were used to estimate the associations with risk factors. Results: We identified 225 EOCRC cases and 88842 controls that include the final analyses. Of the 225 EOCRC patients, 105 (46.7%) were colon cancers and 120 (53.3%) were rectum cancers. EOCRC cases were older, have more intake of fish and eggs, have higher BMIs, diabetes, and family history of cancer compared with controls (P < 0.05). After adjustment for potential confounding factors, increasing age (HR 2.18, 95%CI 2.05-2.31), BMI (HR 1.06, 95%CI 1.01-1.11), family history of cancer (HR 1.41, 95%CI 1.00-1.98), and more intake of fish (HR 1.54, 95%CI 1.09-2.19) were significantly associated with a higher risk of EOCRC. In sensitivity analyses stratified by cancer site (colon and rectum), the results remained consistent. Conclusions: Based on the large Chinese cohort study, we found increasing age, higher BMI or obesity, family history of cancer, and more intake of fish were independent risk factors for EOCRC. Further studies are needed to identify factors that cause the increasing incidence of EOCRC in China and other countries, and explore the potential mechanism behind.