In order to accommodate for analyzing the wide sheet plastic bending in depth according to the strain-increment theory, and to meet the needs of researching on the theoretical solution about thickness variation in metal sheet bending, the concept about the strain-increment neutral layer was projected, the methods for locating the strain-increment neutral layer were discussed, the formula for calculating the strain-increment neutral layer radius ρ_d was established, and the strain\|increment neutral layer and the stress neutral layer were compared. The result shows that the strain\|increment neutral layer and stress neutral layer are two distinct concepts, and need to be rigidly discriminated, otherwise, the wrong conclusion that invariant thickness in the wide sheet plastic bending would be generated.