Modified NUTRIC (mNUTRIC) score is a useful assessment tool to determine the risk of malnutrition in patients on mechanical ventilation (MV). We identified associations between postoperative calorie adequacy, 30-day mortality, and surgical outcomes in patients with high mNUTRIC scores. Medical records of 272 patients in the intensive care unit who required MV support for >24 h after emergency gastro-intestinal (GI) surgery between January 2007 and December 2017 were reviewed. Calorie adequacy in percentage (Calorie intake in 5 days ÷ Calorie requirement for 5 days × 100) was assessed in patients with high (5–9) and low (0–4) mNUTRIC scores. In the high mNUTRIC score group, patients with inadequate calorie supplementation (calorie adequacy <70%) had higher 30-day mortality than those with adequate supplementation (31.5% vs. 11.1%; p = 0.010); this was not observed in patients with low mNUTRIC scores. This result was also confirmed through Kaplan–Meier survival curve (p = 0.022). Inadequate calorie supplementation in the high mNUTRIC score group was not associated with Intra-abdominal infection (p = 1.000), pulmonary complication (p = 0.695), wound complication (p = 0.407), postoperative leakage (p = 1.000), or infections (p = 0.847). Inadequate calorie supplementation after GI surgery was associated with higher 30-day mortality in patients with high mNUTRIC scores. Therefore, adequate calorie supplementation could contribute to improved survival of critically ill postoperative patients with high risk of malnutrition.