Abstract: The stereographic projection is research way through the object three-dimensional space geometry essential factors(line,surface) reflected in the projection plane.The article is a research on application of the stereo-graphic projection method to the directional drilling in the trajectory design and calculation,transforms the space problem as the plane question,carries out computation using the spherical triangle related formula,then obtains a series of characteristic parameter formula.The results show that the design method which unifies the stereographic projection and the spherical triangle analytic geometry is one kind of both simple and the quick methods.