Because of the explicit, the axonometric projection is useful in engineering. But in the published books of mechanical drawing, only the characteristic and the drawing steps of the axonometric projection are focused on, and the forming method and the expression of the unapparent edges of object are not mentioned. This paper analyses the forming method by 3D graphic projective transformation in CG, as well as the unarmed draft of the object. The characteristics of generality about axonometric projection are presented, and the expression of the unapparent edges of object is discussed. These can take supplement to the theory of the axonometric projection.