In the southeast coastal area of China, existing railway lines, constructed long ago with low construction standards for railway subgrades, face significant impacts from the construction of soft ground improvement for newly built railway lines. Through on-site monitoring of foundation treatment construction, it is crucial to select the soft ground improvement method with minimal construction impact to ensure the operation safety of the existing railway. Based on the on-site monitoring data of the excess pore water pressure, surface displacement, and deep displacement in the construction of soft ground improvement, the study assesses the extent and magnitude of construction impact. The key findings include: (1) Ranking of construction impact magnitude: High-pressure jet grouting column (pipe pile) > deep cement mixing column > MJS column > geosynthetic reinforced column; (2) Construction impact range: deep cement mixing column, high-pressure jet grouting column, geosynthetic reinforced column and MJS column are about 5 m, 10 m, 3 m and 4 m, respectively. The construction disturbance of high-pressure jet grouting pile, pipe pile, polymer bag grouting pile and deep cement mixing column are compared based on the differences of excess pore water pressure, surface displacement, deep displacement and construction disturbance range. It is provided a basis for comparison and selection of schemes for soft ground improvement of new railway lines adjacent to existing railways. This optimization effectively controls the construction disturbance of the soft ground treatment of the new line and ensures the safety of the existing railway.