Excavator is important engineering equipment which can be not replaced in the digging stones and soil,the quality diagnosis of key components,intelligent and lightweight are its three development directions. The flatness of high strength steel plate has important influence on the welding quality of excavator working arm,on the basis of traditional multi-roll leveling theory,a stress neutral layer offset model is established based on three-point bending theory,steel plate deforming characteristic and layered theory in the plate leveling process. It proves the existence of the stress neutral layer offset and the relationship of the reverse bending radius,the material thickness and the metal deforming capacity in theory. The mechanical property of high strength steel plate (Q690) is obtained by room temperature tensile test. Combined with the bending test,the neutral layer offset regular is studied in the high strength steel leveling process. The relationship between neutral layers offset values and reverse bending radius is found,the neutral layer offset model is verified and the flatness of leveled Q690 plate is less than 1 mm/m by using multi-roll leveling machine.