Objective To compare the Effects of dexmedetomidine and propofol on diaphragm function and weaning from mechanical ventilation of patients. Methods One hundred and forty patients who received mechanical ventilation were randomly divided into dexmedetomidine group（n = 72）and propofol group（n = 68）. On the basis of conventional treatment and used butorphanol for analgesia，dexmedetomidine and propofol were added to the treatment respectively.Evaluation of diaphragm function by ultrasonography ，The changed of the diaphragm function such as diaphragmatic thickness at the end of expiration ，diaphragm thickening fraction ， diaphragmatic displacement and the Success rate of first weaning and mechanical ventilation time were compared and analyzed. Results There was a significant difference in diaphragm function during first spontaneous breathing trial change between the two groups. The diaphragmatic thickness at the end of expiration，diaphragm thickening fraction and diaphragmatic displacement in dexmedetomidine group were significantly higher than those in propofol group（P < 0.05）. The the success rate of first weaning in dexmedetomidine group were significantly higher than those in propofol group（P < 0.05）. The mechanical ventilation time in dexmedetomidine group were significantly lower than that in propofol group（P < 0.05）. Conclusion Compared with propofol，dexmedetomidine has less effect on diaphragm function in patients with mechanical ventilation，can improve the weaning success rate of mechanical ventilation and shorten the duration of mechanical ventilation.