When taking Ar gas as the working mixture and taking CH4 as the reactive gas, diamond-like carbon (DLC) films are successfully prepared on Ge substrates by using the low voltage reactive ion plating (RLVIP) technology. The microstructures, optical properties and hardness of the prepared DLC films are characterized by means of Raman spectroscopy, infrared spectroscopy and nano-indentation test technology. The result shows that the prepared DLC films (ID/IG=0.918) have a higher sp$^3$ content, a hardness up to 28.6 GPa and a elastic modulus of 199.5 GPa. The single layer film has a peak transparency of 63.6 % and a mean transparency of 62 % in the waveband from 8 μm to 11.5 μm.