During excavation of long and narrow subway station, temporary multistage earthen slope may appear along the longitudinal excavation side. The temporary earthen slope is at high risk of slip failure when it is designed with inappropriate side slope or it is not properly constructed, especially in soft strata. The study aims at a longitudinal slip failure case for a subway station in soft ground and analyzes the comprehensive numerical slope stability with the limit equilibrium method. It turns out that the gradient of Grade I side slope is the major factor to the global stability. The construction slope gradient of 1∶1.75（V∶H）cannot meet the stability requirement of the local slope 〖JP〗in soft muddy substrata, the failure probability is at 88.92%. To ensure stability of the slope in soft strata, a slope gradient of 1∶3（V∶H）or flatter is necessary. In order to prevent the slope instability during temporary excavation, based on the variability of soil strength parameters, the optimal slope gradient is proposed.