In order to deepen the understanding of the unloading effect caused by pre-dewatering before excavation, synchronous monitoring was conducted on the deformation of the diaphragm wall, adjacent ground settlement, and drawdown during the pumping test before excavation based on the excavation project of the subway Station of Nanjing Metro Line 5. Monitoring data show that: The groove shape of ground surface settlement and the bulge shape of lateral wall deflection caused by the pre-dewatering is similar to that caused by the unloading effect of excavation. The influence zones of ground surface settlement caused by pre-dewatering is approximately 3S_w (S_w is drawdown in the pit). The maximum ground surface settlement δ_vm is about 0.033%S_w, and the maximum lateral wall deflection δ_hm is approximately 0.041%S_w. When the groundwater level recovers, there is a phenomenon of ground surface uplift and lateral wall rebound. The ground surface uplift is 32.8%δ_vm at F52. The lateral wall rebound reaches 66.5%δ_hm at ZQT5. The relationship between maximum ground surface settlement and maximum lateral wall deflection is approximately δ_vm=0.8δ_hm. The unloading effect of the pre-dewatering increases the difficulty of controlling deformation during the later construction of excavation. The design of the excavation should review and verify the pre-dewatering conditions.