The damping ratio of soil is an essential and important dynamic parameter when soil dynamic analysis and evaluation. For the diversity and complexity of the hysteretic curve, it is difficult to calculate the damping ratio by the standard method for the excessive amount of data. The simplification of the hysteretic curve is helpful to process the test data quickly. To investigate the characteristics of damping ratio of muddy silty clay in the Yangtze River floodplain in Nanjing under the cyclic load of subway train, the dynamic triaxial test was carried out, and the damping ratio under different confining pressures and dynamic amplitudes was analyzed. For the geometric characteristics of the stress-strain hysteretic curves and the physical significance of cyclic loading, the calculation method is simplified by assembling hysteresis cycles as a group. The results show that with the increase of dynamic strain, the damping ratio shows a three-stage trend, i.e., rapid, slow and stable growth. With the increase of confining pressure, the damping ratio decreases gradually. When increasing dynamic amplitude, the ratio also increases. For the simplified method, the more cycles of the hysteretic loop as a group, the less the ratio. This proposed method can reduce the data treatment work and improve the feasibility of subsequent numerical simulation.