Abstract: Low loss optical coatings mirror is the most critical elements for resonant cavity of ring laser. In this paper, low loss high reflective coatings working at the angle of 45were designed and prepared. In order to improve optical properties of low loss high reflective coatings, high temperature thermal treatment was chosen and the impact of thermal treatment on transmittance, scattering, surface roughness and phase shift of low loss high reflective coatings was researched. The impact of thermal treatment on scattering and surface roughness was very low. With the increase of annealing temperature, the phase shift of low loss high reflective coatings reduced gradually. When the annealing temperature was 550, the optical stability of ring laser output energy was the best. After working for 230 hours, laser output energy dropped to 90.5% of its initial value. The results have shown that optical properties of low loss high reflective coatings and output energy properties of ring laser can be improved by the proper thermal treatment.