Abstract: A beamsplitting film system working in two ultra wide spectral ranges(hightransmissivity at 550～1100nm and highreflectance at 8000～12000nm) is designed. The film system adopts a basic film structure of ｜ZnS｜Ag｜ZnS｜ with a protective coating. With the film system, we obtained a flat splitter which can transmit the visible and nearinfrared bands, and reflect the middleinfrared and farinfrared bands. The actual application shows that the splitter is effective.