The invention relates to the technical field of producing and manufacturing of materials for solar thermal power generation reflectors, in particular to a SiO2-Ag-SiO2 composite membrane for a solar thermal power generation reflector. The SiO2-Ag-SiO2 composite membrane is characterized by comprising a glass substrate and a membrane layer formed on the glass substrate; the membrane layer is composed of a transition layer, a reflecting layer and a protecting layer, and the transition layer, the reflecting layer and the protecting layer are sequentially arranged from the surface of the glass substrate to the outside and all obtained through a radio-frequency magnetron sputtering method; and the transition layer is made from SiO2 and is 20-50 nm thick, the reflecting layer is made from Ag and is 100-200 nm thick, and the protecting layer is made of SiO2 and is 50-150 nm thick. The composite membrane obtained through the method has the characteristics of being high in reflection rate, good in adhesion and long in outdoor service life.