【Abstract】 Objective To explore that different depth can influence incidence of malpositioning from the supine to the lateral decubitus position. Methods Under genral anesthesia,60 adult petients undergoing thoracic surgery were intubated with RobertshawDLTs. To adjust positioning of left double-lumen tube,and randomly assign three groups: Ⅰgroup: In the supine Position, via the right tracheal lumen,the endoscopist should see a clear,straight-ahead view of the tracheal carina, It is important to see the upper surface of the left endobronchial blue cuff just below the carina, then looking down the left endobronchial lumen,the orifice of the LUL bronchus should be seen clearly. Ⅱgroup: The proximal shoulder edge of the blue bronchial cuff should not be visualized at the carina. However,through the left bronchial lumen,and by transparency across the wall of the tube,the position of the tube is adjusted so that the carina is midway between the black radiopaque line and the top of the bronchial cuff.Finally, the orifice of the LUL bronchus and the bronchial carina should be clearly seen.Ⅲgroup: via the right tracheal lumen,the black radiopaque line should be visualized at the carina, the orifice of the LUL bronchus should be seen clearly. Results The incidence of repositioning is significantly less in the Ⅱ、Ⅲ groups compared to theⅠgroup﹙p＜0.05﹚,Ⅱ、Ⅲ are not different﹙p＞0.05﹚. Conclusion Depth can reduce incidence of malpositioning from the supine to the lateral decubitus position in Ⅱ,Ⅲ,suitable depth be situated between Ⅱ and Ⅲ.