Some advances have been made in recent years in the technique for processing three-dimensional depth migration before stack. Nevertheless, as the amount of calculation work for this technique is very great and the precise depth-interval velocity is required, the technique fails to attain the objective of large-scale applicatio. On the basis of applying large scale computer, this paper deals emphatically with the formulation of the model for precise depth interval velocity and suggests the coherent inversion of three dimensional intervl velocity and the optimization of the model for dimensional depth-interval velocity.