Sol-gel method was used to prepare SiO2-GeO2 films. The relative intensity of second-harmonic generation(SHG) signals of the thermally poled films and their relaxations were measured. By comparing the decay of second-harmonic generation of the films with different substrates and under different temperatures,it is shown than the stability of charge near the interface between the film and substrate is influenced by the bulk electric resistance of the substrate,and this charge character consequently influences the SHG stability of the film.