A new technique was proposed to determine the coefficient of thermal expansion (CTE) of thin films at low temperature.Different pre-stress could be applied and the elastic modulus of materials at different temperatures was measured with CTE simultaneously to eliminate the influence of mechanical deformation caused by the pre-stress. By using this technique, the CTEs of polyimide/silica nanocomposite films with different silica doping levels were experimentally studied at temperature from 77K to 287K, and some characteristics related to this new technique were discussed.