The visual sensation is an important factor in urban planning. A computer analysis technology based on the three-dimensional Geographical Information Systems (3DGIS) can be used to measure the visibility in urban space. The principle of sight calculations is introduced at first. The visibility analysis models about terrain and buildings are proposed. In terrain visibility analysis, a method based on projection and elevation interpolation is used to calculate the visibility of two points. And in building visibility analysis, another projection method is proposed to judge the relationship between the sight and building. Based on the visibility analysis of two points, an approximate visual field in a plane can be computed in a discrete way. The visibility analysis models are proved to be feasible. Further research that combines the visibility analysis model and the quantitative planning targets is suggested.