The design and system performance of a far-infrared interferometer with which to test rough aspheric surfaces are presented. It is based on the optical configuration of classical Fizeau interferometer. A CO2 laser is used as a light source operating at a 10.6μm wavelength. A He-Ne laser is introduced into the interferometer to solve problems concerning with alignment. The measuring error(RMS) of the interferometer is less than λ/200 (λ=10.6μm), this accuracy is able to fulfill the need of grinding primary mirror.