The paper is going to give a quantitative computational method for “Landslip Orientation of a body isolated from bed rock in Mountain” only with spatial analytic geometry. The paper gives computational formulae in proper order for only landslip plane, just two landslip planes and just three landslip planes, and gives numerical examples. And the paper gives a general computational model for landslip orientation of m landslip planes. The author puts forward “Un-allowed Straight Angle Law”, “Critical Straight Angle Law” and “Allowed Straight Angle Law”. Finally, the author gives a project expression of a landslip plane on unit sphere.