In order to gain ultra-precision optical elements and to verify the machining capability of ion beam figuring (IBF), the figuring of large optical surface error by ion beam is investigated. Different scale removal functions are gained by changing different ion diaphragms. An 101 mm fused silica flat optical element with initial surface figure peak-valles (PV) value of 417.554 nm and root mean square (RMS) value of 104.743 nm is figured by this mean. Through twelve iterations of 10, 5, 2 mm diameter ion diaphragms, an ultra-precision optical surface with PV value of 10.843 nm and RMS value of 0.872 nm is gained. The result shows that the large optical surface error can be figured by IBF, and the efficiency and precision can also be improved by larger and smaller ion diaphragms.