The influence of diamond film on potassium nitrate infrared absorption was studied. The diamond film was grown on silicon substrates by hot filament chemical vapor deposition (CVD) method. The infrared absorption of potassium nitrate shows that the antisymmetric stretching vibrations ν1 and ν2 of nitrate are red-shifted by 30 and 13 cm-1 respectively. The red-shift of ν1 vibration is mainly attributed to the combination of diamond film with potassium nitrate and phonon resonance effects, and the red-shift of ν2 vibration mode is attributed to the influence of potassium nitrate’s internal stress.