The technological advancement in optical fabrication and testing has exploited a new field for the application of freeform surfaces.Based on the requirements of volume compression and performance promotion for space optics,an initial configuration of a compact,off-axis,three-mirror optical system is presented.The three mirrors are folded,which greatly compresses the volume.In addition,since the nodal aberration theory has laid a theoretical foundation for the application of freeform surfaces,two Zernike polynomial surfaces are adopted in the optimization process.With the verification of a designed example,it can be found that the final optical system possesses the advantages of compact structure,wide field-of-view and small F number,while the optimization process based on this design method can converge to the expected result with a fast speed.