The invention discloses a method for reducing dielectric loss of a niobate-based glass ceramic high voltage capacitor device. Aluminium oxide is added into raw materials of glass ceramics, and mol ratio between added aluminium oxide and silica is 0.1-5%. The niobate-based glass ceramic comprises the following components: xPbO-ySrO-zNb2O5-wNa2O-vSiO2, wherein x, y, z, w and v express mole numbers of components, and x is more than or equal to 5 and less than or equal to 7, y is more than or equal to 9 and less than or equal to 11, z is more than or equal to 30 and less than or equal to 33, w is more than or equal to 15 and less than or equal to 18, and v is more than or equal to 25 and less than or equal to 35. Aluminium oxide is added for improving performance of glass phase, so that in the condition that original preparation processes are not changed, insulation resistance of glass phase is increased, conductance loss of glass phase is reduced, and dielectric loss of the niobate-based glass ceramic high voltage capacitor is reduced macroscopically.