The excavation of large-scale lacustrine soft soil in natural state in the Chengmenshan Copper mine, Jiujiang, China, is a challenging work and is illustrated in this paper. A geometrical model of the lacustrine soft soil was built based on data from more than 150 boreholes. A series of tests were conducted on the natural soft soil including compaction, consolidation and common test, and also remolded samples containing water contents of 10%, 13%, 16%, 21%, 25%, 30% and 37% were tested. Results showed that the cohesion and friction angle decrease rapidly with the increase in water content, but the consolidation pressure played a positive role to the increase in shear strength. To properly manage the soil properties and optimize the surcharge program, settlement and uplift during the infill construction were numerically analyzed by means of SIGMA/W. Based on the test and numerical simulation results, a long term in situ improvement method was proposed and performed. Finally, drilling sampling tests, settlement monitoring and slope stability analysis were conducted to evaluate the overall effects of the surcharge. The properties of the soft soil were improved greatly under the infill surcharge, especially the cohesion and frictional increased by 279.3% and 152.8% and almost reached the value of remolded soil after consolidation under pressure 300 kPa. The settlement rate decreased with the surcharge duration. The safety factor of slope after improvement also met the design requirement. Furthermore, the method proposed in this paper which have given consideration to economic conservation and environmental protection is significant for the soft soil excavation projects.