A Fizeau infrared (IR) interferometer which can test the quality of aspheric surfaces and wavefront aberration of IR aspheric lens is designed and fabricated. The interferometer with aperture of 120 mm comprises F/1 and F/4 standard germanium lens. Two IR detectors are mounted on imaging paths for alignment and measurement, which can be used to monitor orientations of standard lens and elements under test, coupled with raster ruler. The configuration solves the problems of adjustment brought by invisible characteristic of IR light. Phase shifter with great elongation is calibrated by technique of on-line calibration. The quality of IR aspheric lens is measured with the interferometer, and system errors of the interferometer are also tested. Results indicate that the accuracy and repeatability of the interferometer are better than 0.01λ (RMS) and 0.002λ (RMS), respectively.