The invention discloses a build-in length and uplift value calculating method applicable for a narrow foundation pit. The calculating method comprises the step (1) of judging whether a foundation pitis the narrow foundation pit, a supporting structure of the foundation pit comprises support piles and inner supports, if the ratio of the foundation pit depth to the foundation pit width is larger than 1, and the influence of frictional resistance between the supporting structure and soil mass in the pit on the stress and deformation state of the soil mass in the pit prevails, the foundation pitis the narrow foundation pit; and the step (2) of obtaining the build-in length of the foundation pit according to the condition that the unloading stress of the bottom surface position of the foundation pit is equal to the frictional resistance between the soil mass at the position and the supporting structure when the pit is evacuated to the pit bottom in one time. According to the method, the interaction between the supporting structure and the soil mass is introduced; and the method can reflect the influence of the soil mass property on the build-in length and the uplift value fully, and can be applied to foundation pits of any shape.