Polishing is a primary means to get ultra-precision surface in optical manufacturing. In order to obtain the effect of characteristic of pad on surface form of optical flats in polishing, the contacting types between pad and workpiece were analyzed and a model was set up. Then the contacting pressure distribution on the interface between workpiece and polishing pad was calculated by using finite element method. Furthermore, the effect of pad thickness and sphere radius on pressure distribution was investigated. Based on results of theoretical analysis, a new polishing technique for correcting surface form of optical flats is presented. A 430 mm × 430 mm × 60 mm size fused silica workpiece has been polished in the experiment. By conditioning pad surface to convex form and accurately controlling polishing speed ratio, the form accuracy of workpiece has been improved fast.