The invention discloses a tangential knife blade, which is applied to processing rectangular wall surfaces with minimized cutting resistance from the machining surface of workpiece. According to the shape of the tangential knife blade in the invention that the tangential knife blade having a front end face, a back end face, an upper side face, a lower side face, a first mounting side face and a second mounting side face is a hexahedron in general, a through hole passing through the centers of the mounting side faces is provided, and a knife edge is formed at the intersection of each end face and side face; the lower side face has four corners and comprises two tilted sub-surfaces, and a tilted boundary ridge extending between two opposite angles and facing one of another two opposite angles is installed between the sub-surfaces. Therefore, the corner formed by the intersection line of the front end face and the first mounting side face and the intersection line of the front end face and the lower side face sawn from the front end face and the corner formed by the intersection line of the back end face and the second mounting side face and the intersection line of the back end faceand the lower side face sawn from the back end face are the same acute angles in fact.