For the high efficient and high accuracy convergence of surface figure of a large aperture workpiece, the continuous polishing technology was investigated. As the convergence efficiency of surface figure of the workpiece was effected directly by the flatness of pitch lap in a continuous polisher, a high precision measurement equipment with repeatability accuracy of ±1 μm was designed and developed for measuring the flatness of the pitch lap. The change rules and correlation between the flatness of pitch lap and the PV value of surface figure for the workpiece were analyzed during continuous polishing, and relationship between the flatness data and the surface figure of the workpiece was obtained based on the measured data. The experiments indicate that when the diference between flatness and surface curve are larger, the shape of the workpiece is rapidly converged to about 1λ, meanwhile the polishing process transits from a rough polishing to a fine polishing. The high precision measurement equipment proposed by the paper implements precise measurement of the flatness for a large wet pitch lap and provides an important technical support for deterministic continuous polishing.