Abstract: The valve side bushing of the ultra-high voltage converter transformer operates in the multi-field coupling environment of the electric field, temperature field, and mechanical stress field. The capacitor core has an uneven distribution of temperature and electric field. It is urgent to introduce nonlinear characteristics of the insulation medium into the electric thermal coupling mechanism of the valve side bushing and carry out optimization design. This article simulates complex electric field and temperature distribution of the valve side bushing under the excitation of DC and multiple harmonic components, establishes a heating model of valve side bushing capacitor core and proposes valve bushing electric-thermal coupling model considers nonlinear characteristics of capacitor core insulation material. Research has shown during normal operation, the bushing on the side of the ultra-high voltage valve bears harmonic components and DC components in addition to 50Hz power frequency components. After the frequency component reaches 2500Hz, the waveform amplitude approaches zero. As temperature increases in range of 20, 110℃ and the frequency decreases in range of 10^-1, 10⁶ Hz, the tangent value parameter of loss angle increases and there are multiple peaks and valleys. There is a clear nonlinear relationship between the dielectric constant of material and temperature and frequency. The nonlinear model of the valve side bushing electric thermal coupling proposed in the article forms an improved equal margin design method for the electric thermal coupling, achieving the maximum radial field strength of 5.85kV/mm, the axial field of 0.41kV/mm, and the partial discharge margin value of 1.29 for the casing core. The comparison between the calculation and design shows that the axial field strength between the core plates of the valve side sleeve is distributed, which verifies the rationality of the nonlinear model of the valve side sleeve electric thermal coupling. Simulated data in this article can provide theoretical support for manufacturing the ultra-high voltage valve side bushings, and it has certain guiding values for ensuring the safe and reliable operation of ultra-high voltage direct current transmission projects.