The projection of sun direct radiation and the solar radiation intensity gained on wall vary with time; the variation can't be calculated directly in present algorithmic framework of dynamic thermal process in buildings. According to the feature of building thermal process model and the theory of computer graphics, a new geometrical model for analysis of building thermal process is advanced in the thesis. Building in the model is simplified into an aggregation of space regions, which are composed of vector planes. Core arithmetic method for the geometric model is presented, which is vector calculation for straight-lines and planes in three-dimensional space. The geometric model can help to realize the direct calculations of solar direct radiant heat exchanges between walls, and the projection of solar direct radiation; it also has practical value on air flow, mutually shading between buildings, light pollution problems and so on.