Spinel (MgAl<sub>2</sub>O<sub>4</sub>) is an ideal material for infrared window and dome,which plays an important role in infrared imaging terminal guidance. Due to the uneven grain size, the flexible polishing surface of the spinel has grain morphology, which significantly affects the surface quality. In this paper, uniform surface modification of spinel by high-current pulsed electron beam(HCPEB)is investigated. On that basis, flexible polishing is carried out. A new way is provided for reducing and eliminating the grain morphology of the flexible polished surface.<p> </p> Firstly, the mathematical model and numerical simulation for temperature field of spinel modified by HCPEB is carried out. The optimized modification parameters for spinel remelting are obtained: the irradiation energy density is 2J/cm<sup>2</sup> and the irradiation time is 5μs. Then, the spinel is modified on the high-current pulsed electron beam equipment by using those parameters. The change of infrared transmittance is examined on a high resolution spectrometer before and after modification. The modified spinel is polished on magnetorheological finishing machine. The surface morphology and roughness of modified layer is observed on the Zygo white light interferometer after polishing. It is found that the grain is refined and the surface roughness is reduced from Ra 13.28nm to Ra 8.86nm.