The face shape design of progressive addition lenses is based on the original vector height model of the lens surface. The key of the design lies on the choice of the meridian curve of the lens. Based on the analysis of surface shape in gradient zone of progressive addition lenses, accurate orientation of the center of curvature of the lens is made. The influence of the gradient curvature on the lens astigmatism and prism degree is analyzed. A design idea of using involute as the meridian of progressive addition lens is presented. The initial vector height model of the surface of the progressive addition lens is established based on the involute as the meridian in gradient area.