Abstract: In this paper, the flow passing a triangular prism has been calculated by random vertex method. In order to eliminate the oscillation of the tangential velocity at the prism surface,a coefficient Kw is introduced, which is used to consider the interaction of newly generated vortex blobs at the surface. The idea of so called "gravity blob" is proposed to save the CPU time in running the program for simulation of vortex structure behind the triangular prism. The essential features of the large-scale vortex structure in the recirculation region and Karman vortex street downstream are clearly shown in the figures of vortex blob distribution obtained from calculations.