The invention relates to a cassegrain primary mirror precision machining method and an auxiliary fixture thereof. The cassegrain primary mirror precision machining method comprises the following steps: after being clamped and fixed on a seven-axis polishing machine tool, a lens is polished by a polishing head having a micropore structure according to specified parameters. According to the cassegrain primary mirror precision machining method and the auxiliary fixture thereof, the precision of the cassegrain primary mirror surface profile after mechanical polishing can reach an industry-leading value, namely within one wavelength or even one fifth wavelength, and the industrial and streamlined machining is easy to realize.