Objective To explore relationship between body mass index and all-cause mortality or cancer-specific mortality of colorectal cancer. Methods The published articles relevant body mass index and colorectal cancer mortality were retrieved according to the inclusion and exclusion criteria from PubMed, Elsevier-Science Direct, and Web of Science databases. The meta-analysis was performed with RevMan 5.3 software. Results A total of 14 articles were eligible for the meta-analysis, involved 53 804 patients (colorectal cancer patients with underweight 1 853 cases, colorectal cancer patients with overweight 9 088 cases, colorectal cancer patients with obesity 4 463 cases). The results of meta-analysis showed that the colorectal cancer patients with obesity and underweight had the higher all-cause mortalities 〔RR=1.11, 95% CI (1.06, 1.16), P<0.000 01; RR=1.34, 95% CI (1.11, 1.61), P=0.002〕 and colorectal cancer-specific mortalities 〔RR=1.15, 95% CI (1.05, 1.24), P=0.001; RR=1.33, 95% CI (1.09, 1.62), P=0.005〕 as compared with the colorectal cancer patients with normal weight. The all-cause mortality and colorectal cancer-specific mortality had no significant differences between the colorectal cancer patients with overweight and the colorectal cancer patients with normal weight 〔RR=0.96, 95% CI (0.89, 1.04), P=0.31; RR=1.00, 95% CI (0.90, 1.12), P=0.98〕 . Conclusion Colorectal cancer patients with obesity and underweight might have a higher mortality rate.