A measurement method of steel ball surface flaw based on dual wavelength interferometry and digital phase detection is presented. An illumination optical path is designed based on a wavefront matching method. With this optical path the steel ball is illuminated with the max solid angle when the radius of curvature of the incident optical wave is equal to that of the steel ball. And the max solid angle of illumination is dependent on the numerical aperture of the objective lens. Dual wavelength interferometry and digital phase procession are applied to gain the wrapped phase at an equivalent wavelength. The phase ambiguities in the single wavelength phase data are removed to extend the axial measurement range using the equivalent wavelength results. The phase distortion of inclination and spherical aberration is adjusted with application of a phase mask calibrated beforehand. After that high precision three-dimensional data of the steel ball surface can be acquired and applied for surface flaw measurement. Experiments using this technique are shown for the measurement of steel surface flaw and roughness. The results demonstrate the effectiveness of the proposed method.