For high absorption of long-wave infrared spectra, a long-wave infrared ultra-broadband perfect absorber is designed in light of the impedance matching theory by the finite-difference time-domain method in this paper. First, a metamaterial absorber with a metal-insulator-metal structure is analyzed, and the absorber has an average absorptivity of greater than 91% in the range of 7--14 μm. Then, on the basis of the metal-insulator-metal structure, an ultra-broadband perfect absorber with an embedded structure is designed. It has almost perfect absorption characteristics in the range of 7--14 μm with an average absorptivity of over 97.55% and appears to be insensitive to polarization. Its average absorptivity remains over 90% (90.5% in the transverse magnetic mode and 93.7% in the transverse electric mode) when the incident angle is 50°. The research results show that broadband perfect absorption is mainly caused by the combined action of multiple modes such as surface plasmon and the Fabry-Perot resonator. The proposed absorber, with excellent absorption in the range of infrared spectra, holds a promising application prospect in fields such as energy harvesting and infrared sensors.