Zhou, K. JACoW Publishing Geneva, Switzerland The infrared terahertz free electron laser facility of Chinese Academy of Engineering Physics is a multi-functional user facility for frontier research in the fields of materials, spectroscopy, biology, and medicine. Based on the existing terahertz free electron laser facility (CTFEL), by adding two 2×9-cell superconducting accelerators and two undulators, the electron energy is increased to a maximum of 50MeV, the output frequency coverage is expanded to 0.1THz~125THz, and the maximum macro pulse power is greater than 100W. Meanwhile, through the design of the track-shaped beam line, a small energy recovery linear accelerator experimental research platform will be built. This paper mainly introduces the physical design of this facility corresponding to FEL mode and ERL mode respectively.