The sol-gel silica films on K9 glass substrates were prepared with the dip method, and some of them were treated in saturated ammonia gas. Microcosmic surface morphology, chemical structure, refractive index and weak absorption of the films before and after ammonia treatment were respectively measured by Atomic Force Microscopy (AFM), infrared spectrophotometer, ellipsometer and transmission photo-thermal lens. The experimental results indicated that the porous ratio of the films decreased from 0.73 to 0.63 after ammonia treatment, and the weak absorption increased from 67.88×10^-6 to 74.58×10^-6. The Laser-induced Damage Threshold (LIDT) of silica films decreased from 18.0 J/cm² to 16.9 J/cm² after ammonia treatment. Considering the improved mechanical property of films with ammonia treatment, a trade-off is always needed between mechanical property and LIDT according to the requirement in practice.