Abstract: Metalized film capacitors are the core equipment of converter valves in flexible DC transmissions. However, the abnormal temperature rise and bulge deformation problems under the multi-physics due to high harmonic interference and high ambient temperature seriously affect the capacitor's lifetime. This article establishes an electro-thermo-structural coupled model for metalized films, capacitor monomers, and the entire capacitor. The physical field characteristics of the capacitor were analyzed under the harmonic environment of MMC submodules. It is found that the injection of harmonic current enhances the interlayer electric field distortion and ohmic loss of the metalized layer in the capacitor. The superposition of harmonic fields increases the dielectric loss, resulting in an exponential increase of the hot spot temperature and internal stress with the increase of harmonic components. To improve the capacitor's operational stability under harmonic environments, the MOPSO algorithm was used to optimize the structural parameters of the capacitor monomer, and the optimal monomer structure was obtained: the film thickness is 4.7 μm, width is 41.8 mm, and the diameter is 40.6 mm. After optimization, the internal electric field non-uniformity coefficient, hot spot temperature, and maximum stress of the capacitor were reduced by 23%, 22%, and 31%, respectively, which effectively improved its working performance under harmonic environments and provided guidance for its reliability improvement.