The Shanghai-Kunming special railway line for passenger traffic is a high-speed and ballastless railway. The subgrade allows only 1.5 cm of settlement after rail-laying. There is a large quantity of silt distributed along the line, and the designed subgrade settlement is calculated mainly based on laboratory test. The calculation parameter has significant impact on the calculation of subgrade settlement. Typical section of the railway is selected to conduct large-scale plate loading test (PLT) in order to obtain test data, which were further compared with the data of laboratory tests and cone penetration tests(CPT) to analyse the advantages and disadvantages of different test methods and different test results, and also to provide mechanical parameters for the subgrade settlement computation of the silt layer.