As the traditional description of engineering geoinformation is lack of visualization, and can not reveal the spatial tendency of geological structures very well, it is more and more difficult to meet the designers' demands for making geological spatial analysis. By using GIS and visualization techniques, a method of geological digital 3-D modeling and its visualization in hydroelectric engineering is presented in the paper, aiming at making the analysis of geoinformation visual. With an example of engineering application, it has been shown that the method gives an effective way to describe the spatial distribution of geological structures and their complex relationship so that the engineering design can be executed interactively and conveniently.