Objective To investigate the value of resting energy expenditure (REE) monitoring in nutritional support therapy of critical patients on mechanical ventilation.Methods A prospective randomized controlled trial was conducted.Sixty critical patients [acute physiology and chronic health evaluation Ⅱ score (APACHE Ⅱ) ＞ 15] on ventilation admitted to intensive care unit (ICU) of Dalian Friendship Hospital from September 2016 to October 2018 were enrolled.The enrolled patients were randomly divided into Harris-Benedict formula (HB formula) group and indirect energy measurement (metabolic vehicle) group with 30 patients in each group.The HB formula group was used traditional HB formula to determine the energy supply and ratio of nutritional support therapy,and the metabolic vehicle group was regularly measured the energy supply and proportion of nutritional support therapy.Serum albumin (ALB),total protein (TP),lymphocyte ratio,blood glucose,blood gas analysis parameters and REE value were determined at 3,5,7,9,and 11 days of nutritional support therapy.Results The value of REE at 3 days of nutritional support therapy in metabolic vehicle group was significantly higher than that in HB formula group (kJ/d:7 850.4±947.3 vs.6 915.3±875.7,P ＜ 0.05).With the time of nutritional support treatment prolonged,the REE value of metabolic vehicle group was decreased gradually,and after 7 days,the patient's condition was stable and improved,and the REE value tended to be stable gradually,it was significantly lower than that of HB formula group at 11 days (kJ/d:5 046.3 ± 493.3 vs.6 915.3 ± 875.7,P ＜ 0.05).There was no significant difference in blood gas analysis or plasma protein before nutritional support therapy between the two groups.After 5 days of nutritional support therapy,the respiratory function of critical patients in both groups was improved,and the lymphocyte ratio and plasma protein parameters were alleviated.After 11 days of nutrition support therapy,the respiratory function of critical patients in both groups was further improved,the ventilator model was adjusted to continuous positive airway pressure (CPAP) mode,the lymphocyte ratio and plasma protein parameters were improved,and the skin color and elasticity were improved,the granulation of the wound was fresh and healed well,and the plasma protein level was increased obviously,ALB level in metabolic vehicle group was significantly higher than that in HB formula group (g/L:31.8 ± 2.5 vs.26.7 ± 2.3,P ＜ 0.05).In the metabolic vehicle group,REE value was decreased from the maximum level on the 3rd day (k J/d:7 850.4 ± 947.3) to a stable level after 11 days (k J/d:5 046.3 ± 493.3),and its energy ratio changed significantly,from carbohydrate:fat of 77％ ∶ 21％ with protein consumption gradually transition in the early (3 days) to carbohydrates:fat of 56％ ∶ 44％ without protein consumption in the later stage (11 days),which showed the tendency of energy consumption was reasonable.Conclusion The energy metabolism rule of critical patients on ventilation could be determined by using the accurate metabolic vehicle and dynamic monitoring of REE value,which could be used for the implementation of nutritional support therapy.