Abstract: With the advancement of urban renewal, more and more excavations are being constructed under one-side unloading condition caused by the existing adjacent ones, resulting in different deformation characteristics and mechanisms compared to those of the normal excavations. Therefore, with the support of an asymmetrical deep excavation (34.3 m) that is induced by the adjacent one-side unloading in Shanghai, the site monitoring data (i.e. deflection of diaphragm wall, soil displacement, ground surface settlement, struct axial force and earth pressure) are collected and analyzed. The asymmetrical deformation characteristics of the deep excavation induced by unilateral unloading are then summarized, followed by a discussion of the corresponding mechanisms. The results show that the one-side unloading leads to the asymmetrical deformation of diaphragm wall. The displacement at the unloaded side reduces by 76% while that at the non-unloaded side increases by 72%, resulting in an increase of more than one times of the maximum surface settlement at the non-unloaded side. All these changes are the combined results of three mechanisms, including the reduced soil pressure at the unloaded side, the horizontal movement of retaining structures, and the change of the equivalent stiffness of supports in the unloaded direction. Therefore, the asymmetrical deformation of excavations should be fully considered during their design and construction. The equivalent stiffness of supports should be determined appropriately, and the rational monitoring warning indexes should be proposed for the unloading side and non-unloading side, respectively.