IGZO TFT is a typical representative of metal oxide TFT. The basic characteristics of IGZO-TFT devices and the influencing factors of electrical instability were analyzed. In the study of influencing factors， a reasonable explanation was given for the dependence between the film performance of the active protective layer and the electrical properties of TFT. And the preparation method of the active protective layer was optimized through experimental verification， which could solve the problem of poor display of left shift of TFT switch threshold voltage caused by the density of the active protective layer SiO₂. At the same time， the relationship between the characteristics of PECVD SiO₂ films and the important factors of deposition based on IGZO-TFT was also summarized.