We put forword a two-dimensional fictitious sub-network difference scheme first. Based upon it, a computer model is developed to determine topographic parameters including slope, azimuth and screening angle caused by the surrounding terrains at every grid point in mountainous areas, from which the distribution maps of slope and terrain pattern and screening figure can be easily made. The input parameters of the model are altitude data at every grid point derived from the contour map, grid space and number. The grid space may vary according to the contour map scale and uneven degree of the terrains in mountainous areas under study. The test made in the pan of dabie Mountains manifests that the computed values of these parameters are generally consistent with observed ones.