Abstract: DC ultra-high voltage is an important backbone network of China's power grid, and the valve-side bushing of the converter transformer is a key weak link in the ultra-high voltage DC system. The application of digital twin technology to monitor the valve-side bushing is significant. This paper systematically studies the core twin model reduction algorithm in the digital twinning process of valve-side bushing. It proposes a proper orthogonal decomposition reduction algorithm that considers spatial and temporal non-uniformity for ultra-high voltage valve-side bushing. The algorithm balances accuracy and effectiveness well. When the error range of the key study area is less than 0.01%, and the results of the non-key study area can reflect the approximate change of its physical quantity, the time of the reduced order algorithm is only about 5% of that of the full-order simulation model. This study provides an important foundation for constructing a digital twin model for the ultra-high voltage valve-side bushing.