Objective To investigate dynamic changes in the ocular anterior segment in patients with acute anterior uveitis （AAU） using ultrasound biomicroscopy （UBM） and to assess the evaluation of UBM during the treatment of AAU. Methods In this prospective， observational study， 45 eyes from 45 patients with moderate to severe AAU were examined. UBM was performed before （within 5 days of AAU attack） and after treatment of AAU （no anterior chamber cells or vitreous cells were observed by slit lamp microscopy）. Angle opening angle （AA500）， angle opening distance （AOD500）， and ciliary body thickness （CBT） were measured. The results were analyzed by paired t-test. Results Before treatment， the AA500， AOD500 and CBT values in each quadrant of the eyes with AAU were significantly higher than those in normal contralateral eyes （t=3.752， 3.950， 5.277， 6.350， 5.455， P<0.01）. With treatment， the inflammation was controlled gradually 4-6 weeks after AAU onset. AA500， AOD500 and CBT values of eyes with AAU were significantly reduced （t=-16.990， 3.964， 5.373， 6.360， 5.629， P<0.01）， and there were no significant differences compared to normal contralateral eyes （t=-0.189， -0.296， -0.371， -1.693， -0.811， P>0.05）. Conclusion UBM reveals dynamic morphologic changes in the ocular anterior segment structure before and after treatment in patients with AAU. This could provide important information in the development and treatment of AAU.