As scientists' requirements for the polarization accuracy continue to increase, the polarization accuracy of detection equipment becomes more and more important. Therefore, how to accurately measure the polarization characteristics of components and systems in order to optimize and improve the polarization measurement accuracy of the equipment is a key issue. In this paper, a mid-infrared Mueller matrix ellipsometer is designed and established, and the system is calibrated and tested by using the non-linear fitting method and the dual-rotating method at 12.32 μm wavelength. The results show that the measurement accuracy of the Mueller matrix of the ellipsometer is better than 0.02; the repeated measurement accuracy of the transmission sample and the reflection sample is 0.01 and 0.02, respectively. This study can provide reference for the design and application of mid-infrared Muller matrix ellipsometers.