The damping ratio of soil is an essential and important dynamic parameter in 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 and analyze the test data quickly. To investigate the characteristics of the damping ratio of muddy silty clay in the Yangtze River floodplain in Nanjing under the cyclic load of the subway train, the triaxial dynamic 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 the cyclic loading, the calculation method is simplified by assembling the 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 dynamic amplitude increases, the ratio also increases. In the simplified method, the greater the number of cycles in the representative cycle element, the smaller the calculated damping value compared with the standard method. Under the condition of appropriate accuracy, it can be used as a practical method to calculate the damping ratio, so as to reduce the amount of data processing and calculation.