A method for preparing a low dielectric constant (low k) material and a film thereof is provided. The method includes the following steps. A substrate is first put into a plasma generating reaction system, and a carrier gas carrying a carbon and fluorine containing silicon dioxide precursor is then introduced into the plasma generating reaction system, so that the carbon and fluorine containing silicon dioxide precursor is formed on the substrate. After that, the carbon and fluorine containing silicon dioxide precursor is converted to a low k material film through heating; meanwhile, a stress of the low k material film is eliminated such that the film has a more compact structure. By means of these steps the carbon and fluorine containing silicon dioxide precursor is still capable of forming a low k material film of silicon dioxide containing a large amount of fluorocarbon, even under various different atmospheres.