Abstract：Based on Fermat's principle, the paper uses full-path iterative algorithm to realize 3-D VSP two-point ray tracing in volume element model. The volume element model is composed of blocks, in which the interfaces between blocks represent by continuous function or interpolation at discrete points. The initial value is firstly given for the points of intermediate paths during ray tracing, then the matrix equation of revised values is computed by iterative algorithm, which gains the information with satisfied precision at path points. The paper implements forward modeling for acquisition in 3-D VSP geometry and illustrates the adaptability of the method to block model by ray-tracing results of reflection P-wave and P-SV waver on the targets. The ray-tracing precision can be determined according to researched needs. The method can act as auxiliary tool of optimum designing the parameters of 3-D VSP geometry
