Abstract: In the previous seismic analyses of underground structures, only the peak structural response under the main earthquake is considered, but the damage mechanism of underground structures subjected to sequential ground motions has not been fully understood yet. Special attention is paid to the seismic damage analyses of subway station structures under sequential ground motions. The seismic damage evolution of underground structures under aftershocks and the feasible seismic performance indices are explored. Seven bedrock earthquake records, which are back-calculated from seven mainshock- aftershock sequences, are adopted. Considering the influences of the directivity and relative intensities of sequential ground motions, nonlinear dynamic time history analyses of soil-underground structure system are performed. The peak inter-story drift ratios, residual inter-story drift ratios and Park-Ang index are used to evaluate the earthquake damage of the subway station. The results show that the directivity of aftershocks has irregular influences on the structural damage. When the relative intensity between the mainshock and aftershock is large, the underground structures damaged during the mainshock are likely to transit to a severer damage state. In terms of the selection of seismic damage evaluation indexes for the underground structures, the residual inter-story drift ratios are significantly affected by the relative direction between the mianshock and the aftershock. While the peak inter-story drift ratios cannot reflect the damage to the structures caused by the aftershock intensity smaller than the main shock. Therefore, both the peak and the residual inter-story drift ratios are not suitable for representing the additional damage of the underground structures caused by the aftershock. The Park-Ang index can reflect the excessive deformation damage and the accumulated hysteretic energy dissipation, which can better illustrate the actual damage of the structures under the mainshock-aftershock sequences, and is more feasible as the damage evaluation index of the underground structures subjected to sequential earthquakes.