ObjectiveTo explore the value of sarcopenia index (SI) in the diagnosis of malnutrition in colorectal cancer patients.MethodsA retrospective study was carried out to study on 126 colorectal cancer patients who underwent chemotherapy in West China Hospital of Sichuan University between January 2015 and June 2019. SI and body mass index (BMI) were used for malnutrition diagnosis, and the detection rate of malnutrition was compared.ResultsThe detection rate of malnutrition diagnosed by SI (92.1%) was higher than that by BMI (38.1%) with a statistical difference (P<0.001). Subgroup analysis showed: the detection rate of malnutrition diagnosed by SI vs. BMI in male patients was 97.0% vs. 28.4%, with a statistical difference (P<0.001), and that in female patients was 86.4% vs. 49.2%, with a statistical difference (P<0.001); the detection rate of malnutrition diagnosed by SI vs. BMI in elderly patients (≥65 years) was 92.6% vs. 27.8%, with a statistical difference (P<0.001), and that in young and middle-aged patients (<65 years) was 91.7% vs. 45.8%, with a statistical difference (P<0.001).ConclusionUsing SI to diagnose malnutrition for colorectal cancer patients is worth popularizing for it can discover hidden malnutrition patients.