According to the characteristics of a complex cover panel, its geometry shape is describedby the NURBS surface with great description capability. With the reference to the surface classifica-tion determined by Gauss curvature, the proportion of the mid-surface area between before and afterbeing developed is derived from the displacement variation of the mid-surface in the normal vectordirection of the sheet metal during the sheet metal forming process. Hereby, based on the curve de-velopment theory in differential geometry, a novel diagonal point by point surface developmentmethod is put forward to estimate a complex cover panel’s blank contour efficiently. By comparingthe sample’s development result of diagonal point by point surface development method with that ofavailable one-step method, the validity of the proposed surface development method is verified. According to the characteristics of a complex cover panel, its geometry shape is described by the NURBS surface with great description capability. With the reference to the surface classifica-determined by Gauss curvature, the proportion of the mid-surface area between before and after being developed is derived from the displacement variation of the mid-surface in the normal vectordirection of the sheet metal during the sheet metal forming process. Hereby, based on the curve de-velopment theory in differential geometry, a novel diagonal point by point surface developmentmethod is put forward to estimate a complex cover panel’s blank contour efficiently. By comparing the sample’s development result of diagonal point by point surface development method with that ofavailable one-step method, the validity of the proposed surface development method is verified.