A large aperture phase shifting infrared interferometer is developed, which uses a CO2 laser source (λ=10.6 μm) comprising of a Fizeau interferometer with the aperture reaching φ250 mm and an accuracy better than λ/100. And a Twyman spherical interferometer, using a piezoelectric transducer (PZT) to shift a reference mirror and pyroelectric vidican (PEV) as the detector, can also be formed. While phase measurement interferometry (PMI)is carried out, the temperature modulation of the PEV target is also being down. A series of software packages used to fit the wavefront and evaluate image quality are incroporated into the instrument. The testing results of IR telescope wavefront and the flatness of glass mastering discs are presented.