A type of high accuracy infrared spectroscopic ellipsometer, by fixed polarizer, rotating polarizer, sample and fixed analyzer PPr(ω)SA, has been designed and constructed to study the optical properties of infrared materials in the 2.5 - 12.5 μm wavelength range. The ellipsometric parameters ψ and Δ can be derived directly from the detected signal by two ac components with the frequencies of 2ω and 4ω, avoiding measuring the dc component in addition. The system operations, including data acquisition and reduction, pre-amplifier gain control, incident angle, as well as wavelength setting and scanning, were fully and automatically controlled by a computer. The accuracy in straighthrough is better than 1% on tanψ and cosΔ without any defect correction of instrumental elements, which is quite good for the infrared optical constants measurements. Some typical applications on ferroelectric thin films PZT and BST and narrow gap semiconductors Hg1-xCdxTe are presented.