Objective This study was a prospective, single-center, non-randomized controlled trial to investigate the effects of perioperative and out-of-hospital nutritional interventions on nutritional status in patients assessed by nutritional risk screening. Methods From July 2021 to June 2022, a total of 362 patients who met the inclusion and exclusion criteria were enrolled in the Department of Gastrointestinal surgery of the first Hospital of Shanxi Medical University. According to the results of nutritional risk screening and assessment, the patients were divided into two groups: five-step treatment for malnutrition and postoperative routine nutrition support group (FST + PRNS group, n = 248) and postoperative routine nutrition support group (PRNS group, n = 114). The main outcome measures included nutritional risk screening 2002 (NRS2002) and patient-generated subjective nutrition assessment (PG-SGA). Propensity score matching (PSM) was used to deal with confounding factors (age, gender, stage, etc.). Results A total of 302 patients completed the intervention and follow-up. According to the analysis of the study scheme, the NRS2002 score and PG-SGA score of the FST + PRNS group at baseline were significantly higher than those of the PRNS group. At 1 year after admission, the NRS2002 score and PG-SGA score of the FST + PRNS group were close to those of the PRNS group, and the difference was not statistically significant. Conclusion The nutritional status of malnourished patients with gastrointestinal tumors who received one-year five-step treatment for malnutrition and postoperative routine nutrition support was significantly improved, and was close to that of patients with normal nutritional status with gastrointestinal tumors who received postoperative routine nutrition support.