To introduce the ISO10110-5 evaluation index into detecting the surface deviations of optical components, this study uses a plano-convex lens with a large curvature radius as an example, and presents a new method for measuring the surface roundness error of the lens based on the principle of equal-thickness interference. By combining the plano-convex lens with the optical flat to form a Newton's ring instrument, using the eyepiece cross hair of the reading microscope for visual positioning of the plano-convex lens to be measured, and integrating the new reading method of the double-micrometer screw device, the radius variation of each uniform distribution point on the circular interference fringes relative to the starting point is obtained, and the roundness error of the surface cross section of the tested lens is evaluated using the least squares method. The experimental results indicate that the relative uncertainty is approximately 6.5%. The optical techniques used to locate the plano-convex lens can effectively reduce the eccentricity error, and the new reading method also overcomes the limitation of the traditional reading microscope in that the microscope tube can only move in one direction to eliminate space differences. The measurement system does not require special measuring instruments and is simple, practical, low cost, non-contact, and widely applicable.