The preparation of silica aerogelat ambient pressure from waterglass is a promising methodThe gelation process of H2SO4-waterglass system was investigated，and was used to prepare SiO2 aerogel powdersThe results indicate that the pH value had a strong impact on the gelation time of H2SO4-waterglass systemFor the system with constant reactant concentrations，the relationship between gelation time and pH value showed a “W” shape curve，which changed gradually to “U” shape when the reactant concentration decreased continuouslyThe pH value of the sol system with the shortest gelation time decreased in basic condition（pH≥7）and increased in acidic condition（pH＜7）with decreasing reactant concentrationsThe two different gelation mechanisms of H2SO4-waterglass system in acidic and basic conditions should be responsible for this variationAddition of formamide did not change obviously the gelation characteristic of H2SO4-waterglass system，but improved remarkably the physical properties of resultant silica aerogel.