Abstract: Extensive research has been conducted for the seismic response character of the underground engineering. Meanwhile less effect has been put on the seismic input mechanism in the dynamic analysis. To cover this regretful circumstance, this paper makes some discussions on this issue. A rational selection process is firstly proposed for various types of ground motion. The types include original record, modified record, and artificial simulated waves. Design ground motion reference plane(point) and artificial boundary input ground motion determining approach are then studied. A seismic input procedure is formed specifically for underground cavern complex. The seismic input mechanism is methodical and interpreted by integrating the ground motion selection process and seismic input procedure. Ultimately, demonstrations are conducted for the Baihetan underground cavern complex with proposed approaches.