Ultra-thin optical workpieces get significantly deformed when mounted on pressure plates owing to their high aspect ratio and low stiffness, thereby affecting the accuracy in determining the final surface figure. In this study, we mounted a Ф50 mm×1 mm fused silica workpiece using different methods and in a specific curing sequence. The deformation mechanism was analyzed using finite element simulation. The results show that the mounting deformation of the ultra-thin workpiece is asymmetric and irregular and is caused by the curing sequence of the adhesive used instead of the profile of the adhesive surface. With the aim of reducing the effect of the adhesive curing sequence, a mounting method is developed to reduce the mounting deformation from 1.88 μm to 0.51 μm. A flatness of PV 0.46 μm for the fused silica workpiece is achieved post pitch polishing and demounting, and the deterioration is effectively restrained. This study helps to understand the mechanism of mounting deformation and guides the fabrication of ultra-thin optical workpieces.