Objective To apply gastrointestinal ultrasound to evaluate the gastrointestinal function of patients with acute gastrointestinal injury(AGI)and to determine the timing of starting enteral nutrition(EN)therapy to guide clinical enteral nutrition therapy.Methods One hundred and three critically ill patients with AGI level 2(AGI Ⅱ)were prospectively screened at the Department of Intensive Care Medicine(ICU)of the Second People's Hospital of Anhui Province from March 2022 to May 2023,and the following data were recorded,including ultrasound gastric sinus cross-sectional area(CSA),diameter of the descending or ascending colon(CD),peristaltic frequency(CPF),time of EN initiation,prealbumin(PA),EN dose and EN complications.Recovery of gastrointestinal function after EN treatment was judged as successful,and failure was judged if there were complications of EN treatment.Changes in gastrointestinal function after EN treatment were analyzed to determine the timing of enteral nutrition.Results There were 68 cases in the successful group and 35 cases in the failed group.There were no statistically significant differences between the two groups in terms of age,intra-abdominal pressure(IAP),Acute Physiology and Chronic Health Status Score Ⅱ(APACHE-Ⅱ),PA,and disease composition(all P＞0.05).The EN initiation time[(14.71±8.89)h],CSA[(9.24±1.30)cm2]and CD[(2.86±0.41)cm]in the successful group were earlier or smaller than the failed group[(19.52±13.53)h,(10.82±1.96)cm2 and(3.38±0.46)cm](all P＜0.05),whereas the CPF[(2.84±0.96)times/min]in the successful group was faster than thefailed group[(2.32±0.98)times/min](P＜0.05).ROC analysis showed greater value for CSA,CD and CPF to predict EN success,with thresholds of CSA≤9 cm2(AUC=0.892),CD≤2.8 cm(AUC=0.858)and CPF＞3 times/min(AUC=0.744);when the combination of CSA,CD and CPF was predicted to generate PRE_1,the AUC was the largest(0.968)and had the highest predictive value,which could determine the best time to initiate EN.Conclusion Ultrasound monitoring of the cross-sectional area of the gastric sinus,the internal diameter of the colon,and the frequency of colonic peristalsis can predict the efficacy of enteral nutrition therapy in critically ill patients with grade Ⅱ acute gastrointestinal injury and guide the optimal timing of initiating enteral nutrition therapy.