The purpose of this study is to analyze the effect of body mass index (BMI) on patients with concurrent colorectal cancer (CRC) and type 2 diabetes mellitus (T2DM). Patients who underwent primary radical CRC surgery from Jan 2011 to Jan 2020 were retrospectively collected. The perioperative information, overall survival (OS) and disease-free survival (DFS) were compared between the higher BMI group and the lower BMI group. A total of 574 patients with concurrent CRC and T2DM were included in this study. The higher BMI group had higher portion of hypertension (p < 0.01) and coronary heart disease (CHD) (p < 0.01). Furthermore, the higher BMI group had better OS (p = 0.016) and DFS (p = 0.040) than the lower BMI group in stage II CRC. In multivariate analysis, age (OS: p = 0.002, HR = 2.016, 95% CI = 1.307-3.109/ DFS: p = 0.003, HR = 1.847, 95% CI = 1.230-2.772), TNM stage (OS: p < 0.01, HR = 1.667, 95% CI = 1.281-2.169/ DFS: p = 0.001, HR = 1.545, 95% CI = 1.207-1.977), overall complications (OS: p = 0.004, HR = 1.837, 95% CI = 1.218-2.880/ DFS: p = 0.006, HR = 1.783, 95% CI = 1.184-2.686) and major complications (OS: p = 0.005, HR = 2.819, 95% CI = 1.376-5.774/ DFS: p = 0.014, HR = 2.414, 95% CI = 1.196-4.870) were independent factors of OS and DFS. Moreover, BMI (p = 0.019, HR = 0.413, 95% CI = 0.197-0.864) was an independent factor of OS in stage II CRC. Higher BMI was associated with better OS in diabetic patients with stage II CRC.