Single crystal silicon (Si) sphere method is an important scheme for precise measurement of Avogadro constant and redefinition of kilogram. The surface oxide layer thickness is related to the correction of the measured mass and diameter of the single crystal sphere, and contributes a large proportion of the relative uncertainty of the Avogadro constant. We discuss several basic problems in measuring the sphere surface by ellipsometer, i.e., the influence of the crystal- orientation- dependent optical constants and surface curvature induced ellipsometric light beam scattering. And the uncertainty components for the adopted indirect method are analyzed. The study provides both experimental and theoretical bases for the measurement research in surface layer on Si sphere.