Objective:To explore the improvement of diaphragm function after early off-bed mobility intervention in intensive care unit (ICU) patients undergoing mechanical ventilation.Methods:A randomized controlled trial was conducted. A total of 147 adult patients undergoing mechanical ventilation admitted to ICU of Affiliated Hospital of Zunyi Medical University from October 2019 to March 2022 were enrolled. The patients were divided into control group and observation group by convenient sampling. Except for the different intervention programs of early mobility, other treatment and nursing of the patients in the two groups were carried out according to ICU routine. Progressive early activities were performed in the control group, while early off-bed mobility was performed in the observation group. The changes of diaphragm thickness at the end of inspiratory (DTei), diaphragm thickness at the end of expiratory (DTee) and diaphragm thickening fraction (DTF) before and 24, 48, 72 and 96 hours of intervention, and the duration of mechanical ventilation, length of ICU stay and 24-hour re-intubation rate after intervention were compared between the two groups.Results:Among the 147 patients, there were 4 cases of detachment in the control group and 5 cases of detachment in the observation group. Finally, 138 patients were enrolled, 69 cases in the control group and 69 cases in the observation group. There was no significant difference in gender, age, diagnosis of ICU, sedatives, muscle strength, ventilator model, acute physiology and chronic health evaluation Ⅱ (APACHE Ⅱ) score and DTei, DTee, DTF before intervention between the two groups. The DTei, DTee and DTF in both groups were increased gradually with the extension of intervention time, especially in the observation group [DTei (cm) at 24, 48, 72 and 96 hours of intervention in the observation group were 0.247±0.014, 0.275±0.016, 0.300±0.013 and 0.329±0.013, while in the control group were 0.242±0.015, 0.258±0.013, 0.269±0.014, and 0.290±0.017, effect of time: F = 993.825, P = 0.000, effect of intervention: F = 82.304, P = 0.000, interaction effect between intervention and time: F = 84.457, P = 0.000; DTee (cm) of the observation group were 0.213±0.014, 0.227±0.013, 0.243±0.016, 0.264±0.010, while in the control group were 0.213±0.016, 0.218±0.013, 0.224±0.013, 0.234±0.014, effect of time: F = 385.552, P = 0.000, effect of intervention: F = 28.161, P = 0.000, interaction effect between intervention and time: F = 45.012, P = 0.000; DTF of the observation group were (15.98±4.23)%, (21.35±4.67)%, (24.09±4.44)% and (25.24±3.74)%, while in the control group were (14.17±4.66)%, (18.11±3.92)%, (20.22±4.19)% and (20.98±4.12)%, effect of time: F = 161.552, P = 0.000, effect of intervention: F = 49.224, P = 0.000, interaction effect between intervention and time: F = -4.507, P = 0.000]. The duration of mechanical ventilation and length of ICU stay in the observation group were significantly shorter than those in the control group [duration of mechanical ventilation (hours): 112.68±12.25 vs. 135.32±22.10, length of ICU stay (days): 7.84±1.78 vs. 10.23±2.43, both P < 0.01]. However, there was no significant difference in 24-hour re-intubation rate between the observation group and the control group (0% vs. 2.90%, P > 0.05). Conclusions:Both early off-bed mobility and progressive early activities can prevent diaphragm weakness in ICU patients undergoing mechanical ventilation, and the effect of early off-bed mobility is better. Early off-bed mobility can significantly shorten the duration of mechanical ventilation and length of ICU stay, and it is safe and feasible.