Objective To evaluate the effect of potentiated anesthesia of dexmedetomodine when brachial plexus block were unsuccessful . Methods 42 patients with incomplete brachial plexus block were randomly divided into 2 groups, group A Sauteralgyl 25mg and droperidol 2.5mg were used in group A , 0.8ug•kg-1 of DEX were used in group B within 10 min as a bolus, and 0.4ug•kg-1•h-1 of DEX were infused continuously until the end of the operation. Systolic blood pressure (SBP), diastolic blood pressure (DBP), HR, SpO2 ,VAS, Ramsay score and side effects were recorded at before potentiated anesthesia （T0）, 5 min（T1）, 10 min（T2）,30 min （T4） after potentiated anesthesia, end of operation （T5）. Results 3 of group A were turned into group B because of incomplete anesthesia after Sauteralgyl 25mg and haloperidol 2.5mg was used ; compared with group A ,VAS were better in group B at T1, T2, T4，P<0.05；The Ramsay score in group B was better than that of group A at T1~T4，P<0.01；There were no significant difference between two groups in SBP, DBP, HR and SpO2, although there were significant difference among each group. Conclusion DEX can be used to strengthen the effect of brachial plexus block, with good effect of analgesia and sedation, and high security.