Abstract: In order to study the mud pumping mechanism of ballasted track subgrade on the existing lines in China, a physical modelling system which can simulate the cyclic loading-wetting coupling process is developed. The model sample with a diameter of 500 mm is composed of 350 mm-thick subgrade soil and 200 mm-thick ballast. The whole sample is prepared in a high-strength transparent plexiglass model cylinder. The test system is equipped with four types of sensors to monitor the load, displacement, volumetric water content and pore water pressure, respectively. A high-definition camera is installed to observe the whole test process. The developed test system is used to carry out the mud pumping model tests on the soil samples of typical mud pumping disease sections of railway from Xindian to Taishan. The results show that the dynamic pore water pressure is the key factor for mud pumping. As the volumetric water content increases, the amount of migrated particle caused by dynamic pore water pressure gradually increases. In the saturated state, it will cause a large number of particles to migrate, and the phenomenon of mud pumping is significant. At the end of the tests, the ballast fouling index reaches 25%, which will seriously affect the normal operation of the railway in the actual project. It is necessary to replace the fouled ballast.