Ion beam figuring (IBF) Technology to fabricate optical materials with a high thermal expansion coefficient, such as BK7, we were aware that the thermal effect is a troublesome problem that generates high thermal stress. If the thermal stress is over the component mechanical stress limit, the component may crack or break. We discuss this problem in detail and set up a thermal model. Using this model, we gain the machined component temperature field and its corresponding thermal stress field. A filtered IBF method is created to correct the thermal stress field which that gain a more even thermal stress distribution and decrease the thermal stress about 20%.