Three-dimensional (3D) SiO2-Ag porous composite structure substrate was prepared by depositing Ag nanoparticles into 3D SiO2 xerogel via photochemical reduction method. The material has a large specific surface area, high porosity and a high concentration of silanol (Si-OH) on the surface, which endows the material excellent physical and chemical adsorption, resulting in efficient adsorption and capture of VOCs. The SERS enhancement of the Ag nanoparticles allows the substrate to detect target gas molecules with high sensitivity. The substrate has excellent SERS signal uniformity, take toluene gas detection as an example, the relative standard deviation values (RSD) of the main vibrational peaks of 785 cm-1, 1002 cm-1 and 1210 cm-1 at 20 spots are 1.59 %, 1.40 % and 2.08 %, respectively. The long-term stability test showed that the substrate still retained 77.68 % SERS activity after 40 days. The substrate can detect toluene gas as low as 187 ppm. In addition, the SiO2-Ag substrate successfully achieved highly sensitive detection of two and three mixed gases.