In this paper, the slip-softening model of earthquake instability on a strike-slip vertical fault with infinite length has been analysed by the finite element method. The effects of fault parameters on instability and precursors have been researched when there is softening phononous on both stiff and soft regions of fault. Relations between softening low of frictional stress and earthquake prediction, have been analysed, The process of changes in displacement, strain and stress fields before and after instability has been discussed.