Abstract: As urbanization accelerates, long and narrow foundation pits are becoming increasingly prevalent. However, calculating the basal-heave stability of long and narrow foundation pits differs significantly from that of wider ones, and the phenomenon of pit bottom heave is more pronounced in soft soil areas. A long and narrow foundation pit associated with a municipal trench project was analyzed to investigate the force-deformation relationships, basal-heave stability, and sensitivity factors of long and narrow foundation pit support structures. The finite element method, the HSS soil constitutive model, and the strength reduction method were employed to study the effects of excavation width, support structure embedment ratio, surface loading, and support stiffness on the basal-heave stability of long and narrow foundation pits. The results indicated that with higher support stiffness and smaller support spacing, the differences in deformation and force between support points and the midpoint of the supports were minimal. Soil deformation in long and narrow foundation pits was attributed to both bottom heave and ground surface subsidence. A plastic zone formed at the excavation and structure bottoms, eventually traversing the soil body and leading to failure. Sensitivity analysis reveals that the embedment ratio and stiffness of the support structure exert a pronounced positive effect on the basal-heave stability of narrow excavations, whereas excavation width and adjacent surcharge show a negative correlation, with the embedment ratio displaying the strongest correlation. These findings provide a scientific basis for the design of support systems and stability assessment of narrow excavations in soft soils.