The invention discloses a method for producing porous nano silica and active carbon by utilizing rice hull ash, which comprises the following steps of: at lower temperature and normal pressure, performing secondary alkali dissolution on subcarbonate, and filtering to obtain alkali-dissolved filter liquor and filter residue; adding a certain amount of dispersing agent into the filter liquor, reducing temperature for ageing, filtering, washing, and removing the dispersing agent to obtain the porous nano silica; and adding an activating agent into the alkali-dissolved filter residue, activating, washing and drying to obtain the high specific surface area active carbon. The alkali and the active carbon can be recycled, and only a small amount of alkali and active carbon is lost. The method has the advantages of not seriously corroding equipment due to no high-temperature and high-pressure alkali boiling, sufficient raw materials, low price, no environmental pollution, and higher quality of the obtained active carbon and silica.