The steel plate straightening quality has an important influence on the welding quality and service performance of excavator's boom and arm. In this study, a stress neutral layer offset model was established based on three-point bending theory, steel plate bending characteristics, and layered theory in the straightening process. The stress neutral layer offset phenomenon and the relationship among the reverse bending radius, the plate thickness, and the metal deformation capacity have been proved theoretically. Combined with the tensile and bending tests at room temperature, the relationship between the neutral layer offset values and the reverse bending radius was obtained, and the results showed that the stress neutral layer offset value increases with the decrease in the reverse bending radius. The neutral layer offset phenomenon was also verified by the straightening experiment, and the flatness of the straightened high-strength steel plates is less than 1 mm/m.