Objective To investigate the correlation between serum carnitine levels and sarcopenia and nutritional status in colorectal cancer patients undergoing radiotherapy. Method A total of 84 patients with colorectal cancer radiotherapy who were treated in our hospital from January 2020 to December 2021 were selected as the case group including 42 cases with serum carnitine level ≥39. 77 μmol/ L high level group and serum carnitine level was lower than 39. 77 μmol / L are 42 cases low level group . and 61 cases of healthy subjects during the same period were selected as the control group. Serum free carnitine levels were determined by enzyme cycle method and serum free carnitine levels in the case group and control group were analyzed. The nutritional status and muscle status of the two groups in the case group were analyzed Pearson correlation was used to analyze the correlation between serum carnitine levels and sarcopenia and nutritional status in colorectal cancer radiotherapy patients. Result The serum free carnitine level in the case group was significantly lower than that in the control group P<0. 05 . In the case group the serum carnitine levels walking speed hand grip strength prealbumin femoral neck BMD hemoglobin Hb albumin ALB total protein TP skeletal muscle index SMI and lean body tissue index LTI were significantly different P<0. 05 Pearson correlation analysis showed that serum carnitine levels were significantly associated with SMI r = 0. 676 femoral neck BMD r = 0. 544 walking speed r = 0. 560 hand grip strength r = 0. 609 LTI r = 0. 411 TP r = 0. 532 prealbumin r = 0. 532 ALB r = 0. 623 and Hb r = 0. 376 were positively correlated P<0. 05 . Conclusion The serum carnitine level is highly expressed in patients with colorectal cancer radiotherapy which can be used as a serum marker for sarcopenia and nutritional status in patients with colorectal cancer radiotherapy.