With the increasing application of nonlinear electrical equipment in the industrial field, the harmonic content in the power grid is gradually increasing, which brings great harm to the foil-film capacitors used in the system. In order to study the breakdown characteristics of the dielectric in the foil-film capacitor under DC superimposed harmonic voltage at different temperatures, a breakdown test platform is established for the foil-film capacitor immersed in benzyl toluene and the foil-film capacitor elements are subjected to DC voltage, DC superimposed 3rd harmonic voltage, DC superimposed 5th harmonic voltage at 40°C, 55°C, 70°C and 85°C respectively. The results show that under the same type of voltage, the breakdown strength of the insulating medium in the foil-film capacitors decreases with the increasing temperature. Besides, in the temperature range of 40°C ~85°C, the breakdown strength of the insulating medium of the foil-film capacitors decreases with the increase of the harmonic order of the harmonic voltage at the same temperature.