Objective To investigate the value of quantitative computed tomography (QCT) body composition parameters in assessing the prognosis of patients with resectable rectal cancer (RC). Methods A total of 94 patients with colorectal adenocarcinoma from 2016 to 2021 were retrospectively collected. All patients underwent QCT scan before operation. Skeletal muscle cross-sectional area, total fat area, subcutaneous fat area and visceral fat area were measured, and skeletal muscle index (SMI), total fat tissue index (TATI), subcutaneous fat index (SATI) and visceral fat index (VATI) were calculated. TATI, SATI, and VATI truncation values were obtained to predict postoperative outcomes in RC patients, and sarcopenia (SA) was evaluated by SMI. Results The cut off values of TATI, SATI, and VATI were 106.39 cm ²/m ², 47.33 cm ²/m ², and 42.61 cm ²/m ². The overall incidence of SA was 30.0% (28/94). Tumor N-stage, SA, and VATI were independent risk factors for postoperative outcomes in RC patients ( P < 0.05). The AUC of the nomogram model was 0.844 (95% CI: 0.755 - 0.911). Conclusion Tumor N-stage, SA, and VATI correlated with prognosis in RC patients undergoing surgery. The nomogram model established in this study has some value for evaluating the prognosis of patients with resectable RC.