In order to describe the pore scale fractal distribution, we present a 3D network model for fractal porous medium in this paper. According to the proposed model we simulate the immiscible two-phase flow displacement in a medium and study the viscous fingering pattern of displacement front influenced by the fractal dimension Df for pore size distribution and viscosity ratio M. The simulation results show that the capacity dimension Dh of the viscous fingering pattern decreases with Df and M increasing, and their quantitative relationship is derived by fitting the obtained data.