Aiming at the state of affairs that the backside soil of the basis pit in the tender soil location is saturated smooth clay, the stability of narrow foundation pit under different insertion ratios is studied based on the slender foundation pit of a cable tunnel in a city of Fujian Province. Centrifugal model tests with three different insertion ratios were designed to simulate narrow foundation pit excavation, and the soil displacement and stable failure mode of foundation pit were observed. The results show that: When the insertion ratio is 1∶1 and 1∶0.8, the deformation of the retaining wall is small, and the deformation of the retaining wall indicates a drum-shaped deformation mode of "small at each end and giant in the middle. Whe insertion ratio is 1∶0.8, compared with the situation of insertion ratio is 1∶1, the deformation of the retaining wall enlarges slightly, which can meet the deformation requirements. It is more economical rationl when the insertion ratio is set to 1∶0.8. When the insertion ratio is 1∶0.67, the width effect of narrow foundation pit is lost due to the insufficient penetration depth of retaining wall. The retaining wall reveals an inverted "八" structure deformation pattern, a massive quantity of soil at the backside of the pit uplifts, and the basis pit is destroyed. Finally, the numerical simulation method is used for assessment and verification. The lookup consequences of this paper can notably optimize the insertion depth of slender basis pits, thereby decreasing engineering costs, saving development resources, and enhancing aid utilization efficiency.