The deformation mechanism of a soft soil shield tunnel in Shanghai adjacent to a foundation pit under the effect of unloadingloading of foundation pit is studied; the influences of the supporting scheme for underground diaphragm wall and the clear distance between shield tunnel and foundation pit on shield tunnel structure are analyzed. The study results show that: 1) The displacement of shield tunnel towards foundation pit would be induced by unloadingloading effect when the foundation pit is on the side of shield tunnel. The vertical displacement and horizontal displacement of shield tunnel would be recovered with the loading time increases. 2) The maximum vertical displacement of shield tunnel would occur nearby corner of the foundation pit and the maximum horizontal displacement of shield tunnel would occur at 1.5h(h is the height of foundation pit) point (from tunnel axial line to foundation pit) under the effect of free surface scope of foundation pit and unloadingloading. 3)The horizontal displacement of shield tunnel would be reduced by 50% by setting underground diaphragm wall. 4) The unloadingloading effect of foundation pit can be ignored when the clear distance between shield tunnel and foundation pit is larger than 1.5h.