A DC superimposed harmonic aging test platform was established to address the aging problem of DC-link capacitors in the flexible DC converter under actual operating conditions. By applying different parameters of DC superimposed harmonics and pure DC voltage to the capacitor components, the aging life of the capacitor under different electrical stresses was obtained, and then the aging curve of the capacitor was obtained. By comparing and analyzing the attenuation of capacitance between two groups of capacitors, the attenuation law of capacitor life under DC superimposed harmonics was obtained. Subsequently, through further variable control experiments, the important influencing factors of harmonic induced aging of DC capacitors were obtained, providing a theoretical research basis for the life assessment of DC capacitors in the flexible DC converter.