The width and depth of the foundation pit of Zhangcha Station on Foshan Metro Line No. 2 are 50.3 m and 16.9 m respectively. Part of the pit is located under the existing Chanxi Road Bridge with clear height of 7 m. In order to solve the problems of support of superwide deep foundation pit in low space, pile foundation underpinning of existing viaduct, etc., a series of technical measures are proposed, as follows. (1) Pile foundation underpinning of highrise pile cap technology is used to underpin the bridge pile in the middle station, and the underpinning cap is higher than the surface of station pit; the inner support of the foundation pit passes through the existing and new pile foundation to form a symmetrical support; and the inner support is relatively independent of the existing and new piles. (2) The width of the diaphragm wall is adjusted to 4 m; and the small drilling rig is used to cut the groove to improve the construction technology under the bridge. (3) The measures of increasing the anticollapse hole between the diaphragm wall and the existing piles on both sides is proposed. (4) The inner support of foundation pit adopts concrete support and temporary columns to increase the stability of inner support. The measures adopted have solved the mutual influence of underpinning system and foundation pit, which can ensure the safety of superwide and deep foundation pit in low space and the safety of the existing pile foundation. The numerical calculation and field construction indicate that the key underpinning technology for pile foundation of existing viaduct in superwide and deep foundation pit is rational, safe and feasible.