<p>Full-aperture continuous polishing is one of the key processes for polishing large flat optical elements. The surface figure of the optics polished by continuous polishing depends largely on the pitch polishing lap’s surface shape, which is controlled by a large conditioner. The changing process of the surface shape of the viscoelastic pitch lap under the loading conditioner is extremely complicated and has not been fully understood. This paper focuses on the formation mechanism and changing principle of the dynamic shape of the pitch lap during polishing. Firstly, the viscoelastic properties of the pitch lap are analyzed, and a viscoelastic creep model of the pitch lap is established. Then, the dynamic alteration process of the surface shape of the pitch lap under the loading pressure of the conditioner is explored, and a solution model of the dynamic shape of the pitch lap at the polishing station is established. Finally, polishing experiments were conducted to verify the model. This paper for the first time reveals that the surface shape of the pitch lap presents a dynamic alteration, which lays a foundation for controlling the surface shape of the pitch lap.</p>