Full-aperture continuous polishing is crucial for refining large flat optical elements， with the main challenge being the deterministic control of the elements' surface figure. This study delves into the factors and control strategies affecting the surface figure to enhance the process's deterministic control. It emphasizes the role of kinematic parameters， the polishing lap's surface shape， and its passivation state. A kinematic model， based on effective sliding distance， was developed to highlight the impact of the lap's groove design on the surface figure. A novel approach for measuring the lap's surface shape involves using a displacement sensor for scanning， followed by shape reconstruction through an interpolation algorithm. The paper introduces an in-situ method for precise correction of the lap's shape error using a subaperture technique with small tools. In addition， it proposes a monitoring method for the lap's surface passivation state and explores its effect on the surface figure. Findings indicate that annular grooves tend to produce annular textures， whereas square and spiral grooves yield smoother surfaces. Through in-situ measurement and correction of the lap's shape error， significant enhancements in surface figure can be achieved. The surface deteriorates as the lap becomes more passivated. An 800 mm×400 mm×100 mm flat element polished with this method achieved a surface figure better than λ/6. This research sheds light on the factors influencing surface figure in full-aperture continuous polishing and introduces a quantitative control approach， significantly enhancing the process's control precision.