The connotation of bridges crossing the active fault (BCAF) was discussed in terms of time scale, fault activity and site effect. The great earthquake damage risks BCAF has to face was analyzed, and the reasons are summarized as "function" and "restriction" and so on. Based on the strong motion records of Imperial Valley earthquake collected by the El Centro array in 1979, the characteristics of strong earthquakes and the mechanism of earthquake ground motion of fault-rupture were analyzed, the ground motion distribution law on both sides of fault was studied, and a suggestion is put forward for the ground motion input direction of bridges crossing the strike-slip fault. The research results have reference significance for the promotion of seismic fortification of BCAF.