The dynamic characteristics of subgrade is aggravated under the interaction of immersed infiltration and dynamic loads of trains, which affects the safety of train operation and long-term stability. Based on the engineering background of cement-stabilized expansive soil subgrade of Hao-Ji heavy-haul railway(Haolebaoji-Ji'an), field excitation tests of subgrade with four million cycles were carried out under dry and immersed conditions to investigate the dynamic characteristics. Large-scale excitation equipment, combined with dead weights, were used to simulate the dynamic behavior of heavy-haul trains with 25-30 t axle load. The test results show that the variations of dynamic stress and acceleration along the depth of subgrade are consistent, and decay rate is 80% at the base of subgrade. The influence of immersed infiltration and dynamic load of train is more significant at the interface of between the subgrade bottom and fill. Under the same loading conditions, the dynamic stress at the interface under the immersed condition is 28% larger than for the dry condition. The acceleration is much less sensitive to the immersed environment than the dynamic stress. At the same time, the dynamic stress level along subgrade depth is much lower than the critical dynamic stress of fill in the same location. The cumulative deformation of subgrade surface under cyclic loading of 4 million times is less than 5 mm and remains stable, which indicates that the improved expansive soil with 3%-5% cement content can be used as the subgrade bottom and fill to meet the dynamic stability requirements of subgrade. The research results can provide theoretical reference for high-quality construction and maintenance of the improved expansive soil subgrade of heavy-haul railway.