Based on the seismic fragility method， this paper designed a random sampling procedure to investigate the effect of differ? ent amounts of input ground motions on the seismic fragility curves of underground structures in the cloud method. Taking a shal? low-buried subway station with two-story and three-span as the research object， a non-linear dynamic interaction finite element model of the soil-underground structure was established， and 350 natural ground motions were selected as inputs to calculate the seismic response. The seismic fragility curves were constructed based on PGA-IDR finally. The results show that the amount of in? put ground motions has a greater effect on the PGA thresholds for each performance level of underground structures when the amount is less than 190. When the amount is greater than 190， the seismic fragility curves for minor and moderate damage to under? ground structures are not affected by the amount； when the amount is greater than 280， the curves for extensive damage and col? lapse are negligibly affected by the amount. When the underground structure is in an area with a low probability of strong seismic threat， this paper recommends 190 natural ground motions as inputs， otherwise， it should be 280.