The vibration/rotation modes and the corresponding infrared spectra of deuterium molecules with the D2d/D2h structures are calculated using the coupled cluster singles and doubles method with the cc-PVTZ basis sets within the framework of the density function theory. The infrared (IR) absorption spectrum of liquid deuterium is experimentally measured in a home-made cryogenic target system by a home-made low-temperature infrared spectroscope. The experimental results show that the strongest IR absorption peak of liquid deuterium is related to the Q1(0)+S0(0) mode, which is in good agreement with the theoretical calculation.