Excavation unloading of foundation pit will lead to additional stress in adjacent subway tunnels. Excessive additional stress will cause instability of subway tunnel, and then affect the safety of subway operation. In order to study the influence law of additional stress on tunnels, the calculation formula of tunnel additional stress caused by foundation pit excavation is derived by using Mindlin theoretical solution and is verified based on the excavation project of a foundation pit group near a subway tunnel in Hangzhou. Then, the influence of three factors, including tunnel depth, horizontal distance between tunnel and foundation pit, and angle between tunnel and foundation pit, on additional stress in tunnel is analyzed. The research results show that: (1) with the increase of tunnel depth, the vertical additional stress of tunnel increase while the horizontal additional stress decrease, and the vertical and horizontal additional stress will decrease with the increase of the horizontal distance between tunnel axis and foundation pit; (2) during excavation of foundation pits, with the increase of horizontal distance between tunnel and foundation pit, the influence coefficient of tunnel depth on vertical additional stress decreases gradually, while the influence coefficient of tunnel depth on horizontal additional stress decreases first and then increases; (3) the vertical and horizontal additional stresses increase with the increase of the angle between tunnel axis and foundation axis, and the additional stress of the tunnel is mainly caused by excavation of the foundation pit which is the closest; (4) with the increase of the distance between tunnel axis and foundation pit axis, the reduction rate of vertical additional stress caused by pit group excavation is smaller than that caused by single pit excavation, while the reduction rate of horizontal additional stress caused by pit group excavation is larger than that caused by single pit excavation.