The key dwell time solution algorithm in ion beam figuring (IBF) was investigated to get the high-precision optical surface. With the principle analysis of the IBF process,the deconvolved process of dwell time solution was transferred to a dwell time matrix equation. Except the regularization weight faction,another variable,that is extra removal amounts,was introduced to the dwell time matrix equation to expand the freedom of dwell time solution,so the dwell time solution could be found with larger range. With the Gerchberg band limited extrapolation algorithm for initial surface expansion,the high-precision surface could be gotten within full aperture range. The example calculation of 50 mm planar optical surface shows that the final surface precision was reduced to:rms is 0.001λ,PV is 0.0115λ from the initial rms of 0.5747λ,PV of 2.3706λ(λ=632.8 nm). The conclusion is that the perfect dwell time solution could be gotten from the revised dwell time matrix equation and initial surface expansion,and so it can be used to instruct the IBF process perfectly.