Using a home-made cryotarget system and infrared spectroscopy system, we measure the infrared absorption spectra of liquid hydrogen at low temperaturs. It is found that the fundamental vibration frequency of hydrogen molecule is 4237.3 cm-1. As liquid hydrogen cools from 19.75 K down to 16.75 K, the infrared absorption is enhanced. The vibration frequency of hydrogen molecules is calculated on the basis of non-harmonic approximation in order to explain the experimental ones.