The SiO₂ aerogel samples were obtained via one-step sol-gel route and ethanol supercritical dried method. A silicon oxide precursor, tetramethyl orthosilicate (TMOS) was dissolved in methanol and hydrolyzed. By using ammonia as a catalyst and methanol as solvent, SiO₂ aerogels with the density of 95, 110, 135, 165 mg/cm₃ were prepared. Combining with 635 nm wavelength laser and 90° direction scattering of samples, the microstructure-property relationship was studied by analyzing the results of small angle x-ray scattering. In this paper, with the increase of initial solvent, density (r ) and mass fractal dimension (Dm) decreased, while the primary particles (a) and secondary particles (d) increased. The reason may be that the phase separation is changed from the nucleation-dominated to the growth-dominated for initial concentration of TMOS decreased. In addition, the primary or secondary particles were assumed as the scatterer. The results are analyzed, respectively. It is revealed that interaction between the aerogels and the light is irrelevant Rayleigh scattering, and the main scattering took place on the secondary particles. The scattering is determined by the secondary structure, including the fractal dimension (D), the size of primary particles (a), and the size of secondary particles (d).