Abstract: According to the deflection deformation equation of rectangular beam in material mechanics, a new formula for calculating residual stress of the rectangular section strip specimen by layer cutting method was derived. The residual stress of 7050-T7451 aluminum alloy rolled plate with a thickness of 165 mm was calculated by this formula, and the results calculated by this formula were compared with the results calculated by existing standard formula. The results show that when the milling layer number was not more than 9, the relative error between the residual stress calculated by this formula and the existing standard formula was less than 6%, indicating that this formula was accurate and could be used to calculate the residual stress of metal plate.