The optothermal spectrum of infrared MPA of CF3CDC12 obtained by using optothermal cell with a platinum filament as a detector is reported here. It is found that the absorption peak at 944 cm-1 in infrared linear absorption spectrum splits into two peaks at 947 cm-1 and 927cm-1 in MPA spectrum. This phenomenon could be related to-the coupling of normal vibrations produced by Coriolis forces and anharmonisity. So one could select a more suitable excited wavelength according to this new result in laser separation of deuterium isotop. The absoption peak at 986 cm-1 in infrared linear absorption spectrum shows a red shift of 6cm-1 in MPA spectrum. This is due to the anhamonicity of vibrational levels of the molecule.