Researching the long side effect of the long and narrow foundation pit in Foshan soft soil area is of great significance for the safety construction of such foundation pits. With the long and narrow foundation pit of a Metro station in Foshan as research background, we simulated the long side effect by finite element software in association with monitoring data of underground diaphragm wall. Results implied that the long side effect of long and narrow deep foundation pit is detrimental to construction safety by increasing the maximum horizontal displacement of underground diaphragm wall in the long side span and lowering the depth of maximum horizontal displacement. The simulation results are relatively conservative, apparently smaller than measured values. Increasing the stiffness at mid-span of the long side of foundation pit could alleviate the long side effect for the safety of construction period.