Abstract: The previous researches calculated the curvature by using the least square method to approximate the local surface with 3×3 grid. Its specific calculation formula is deduced by Roberts. The aperture brings a lot of noise when we calculate the value of the curvature. In order to solve this problem, we use the least square method to approximate the local surface with 5×5 grid, give the detailed calculation formula and then compare with the results got by 3×3 grid, 5×5 jumping grid and 5×5 grid. The results prove that the improved curveture methods has enhanced the anti-noise ability.