The invention discloses a digital helical surface envelope method for calculating a forming tool silhouette by means of an already-known helical surface and relates to the helical surface product forming tools. The method comprises the steps of determining a structure and parameters, namely a helical surface equation of the helical surface of a workpiece; establishing a space coordinate relationship between the helical surface of the workpiece and a forming tool, and solving a coordinate transformation matrix of the space coordinate relationship and an expression of the helical surface of the workpiece under a forming tool coordinate system; according to the movement relation between the workpiece and the forming tool, determining an envelope movement track of the helical surface of the workpiece, and obtaining an equation of a helical surface cluster; in graphic software developed based on OpenGL, drawing the helical surface cluster, obtaining the cross section shape, namely forming tool silhouette, in a certain axial plane of the forming tool, of the helical surface including the surface cluster, extracting data of the cross section shape, and completing the design of the forming tool silhouette. The traditional forming tool design through the analytic computing theory can be replaced with the method, only the movement relation between the helical surface and the forming tool is needed to be defined, and the forming tool silhouette corresponding to the helical surface under the movement relation can be obtained.