Objective: To investigate the relationship between changes in ultrasound echo characteristics of the diaphragm and peripheral skeletal muscles and weaning outcome and prognosis in mechanically ventilated patients with sepsis, in order to explore the potential value of muscle ultrasound in guiding the early and accurate assessment of patients with sepsis. Methods: Mechanically ventilated septic patients admitted to the Department of Critical Care Medicine of Affiliated Hospital of Nantong University from January 2023 to December 2024 were selected as the study subjects, and the ultrasound images of diaphragm and quadriceps femoris were consecutively observed and analyzed at the time of admission to the ICU, 48h, and discharge from the ICU. The echogenicity of diaphragm (EDi)/echogenicity of rectus femoris (ERF) ratio (EDi/ERF) and the thickness of diaphragm (Tdi)/quadriceps muscle layer thickness (Q-MT) ratio (Tdi/Q-MT) were calculated. According to the trend of EDi/ERF and Tdi/Q-MT, the patients were categorized into the group with increasing ratio and the group with decreasing ratio. The weaning success rate, duration of mechanical ventilation, length of stay, and 28-day survival rate of the two groups were compared separately to evaluate the guiding value of changes in the ultrasound characteristics of the diaphragm and peripheral skeletal muscles on the clinical outcome of mechanically ventilated patients with sepsis. Results: A total of 110 patients were enrolled in the study, and the patients in the group with increased EDi/ERF ratio had a significantly lower weaning success rate(P=0.043), a significantly longer mechanical ventilation time(P=0.019), and a significantly lower 28-day survival rate(P=0.008) than those in the group with decreased EDi/ERF ratio. No statistically significant differences in weaning success rate, duration of mechanical ventilation, and 28-day survival rate were found between the group with increased Tdi/Q- MT ratio and decreased Tdi/Q-MT ratio (P>0.05). The Cox proportional hazards model showed that enhanced EDi and increased EDi/ERF ratio were significant risk factors affecting the 28-day cumulative weaning success rate (HR=0.306, P=0.045; HR=0.287, P=0.031, respectively). Conclusion: The early presence of increased echogenicity of diaphragm and increased diaphragm to peripheral skeletal muscle echogenicity ratio are both associated with lower weaning success rate and poorer clinical prognosis in mechanically ventilated patients with sepsis.