Objective ·To investigate the morphologic and functional changes of the diaphragm of children during early mechanical ventilation under synchronized intermittent mandatory ventilation (SIMV) mode. Methods ·Children were admitted to the Pediatric Intensive Care Unit (PICU) of Xinhua Hospital, Shanghai Jiao Tong University School of Medicineand received mechanical ventilation for at least 96 h from October 2020 to December 2021. Bedside ultrasonic testing was conducted to inspect the changes in the diaphragm at three different time points (0, 48, and 96 h, respectively). Kruskal-Wallis H test was conducted for examining the abdominal fat thickness, atrophy rate, diaphragm thickness and DE between the groups. The diaphragm thickness was further compared by Bonferroni's test for ex-post hoc comparisons. Univariate ANOVA was performed for analyzing the diaphragm contraction velocity, diaphragm thickening fraction and diaphragmatic atrophy rate between the groups. Results ·Forty-six children in PICU with complete measurement data were included. Their average age was 2.94 (1.35, 7.00) years, including 23 males and 23 females. The main disease leading to perform mechanical ventilation was pneumonia (52.17%). There was no significant difference between the choice of ventilator parameters and oxygenation status during the observation period (P>0.05). In the early stage of mechanical ventilation (within 96 h), 50% of children showed atrophy of abdominal subcutaneous fat, 36.96% showed nutritional disorders (fasting and need parenteral nutrition support), and 93.5% of children received glucocorticoid therapy. However, there was no significant difference in the atrophy degree of abdominal subcutaneous fat between the three time points (all P>0.05). Bedside ultrasonography detected significant atrophy of bilateral diaphragm thickness at three different time points (all P=0.000). After 48 h of mechanical ventilation, the atrophy rate of the right diaphragm was 4.27%±7.36%, and the left diaphragm was 3.88%±6.85%. After 96 h of ventilation, the atrophy rate of the right diaphragm was 7.69%±7.74%, and the atrophy rate of the left diaphragm came to 7.55%±7.69%. The atrophy rate of the bilateral diaphragm was significantly higher in the first 48 h of mechanical ventilation than in the 48 h to 96 h (all P =0.000). However, there were no statistically significant differences in changes of diaphragm function-related indicators (diaphragm excursion, diaphragm contraction velocity, and diaphragm thickening fraction). Conclusion ·The bedside ultrasound is a proven tool to detect diaphragmatic atrophy in mechanically ventilated children. Mechanical ventilation can induce structural atrophy of the diaphragm in children during early mechanical ventilation under SIMV mode, which is more pronounced in the first 48 h. However, the early morphological changes of the diaphragm have not affected its function.