"Step" is the main physical model for the interpretation of gravity gradient zone anomaly. The theoretical calculation has proved that the Bouguer gravity anomaly (Δg) directional derivative (V_xz) peak value indicates the ground projection position of the top edge of the step that forms the gravity gradient zone, but is less related to the angle of the step. Using the 3/4 extreme value position, the authors deduced another inversion formula and proved that this formula is better than the common formula which uses 1/4 extreme value position.