Transformer bushing, as an important insulating accessory of transformers, plays a critical role in ensuring the safe and reliable operation of transformers. Compared to traditional oil-immersed bushings, resin-impregnated fiber (RIF) bushings have been in operation for a relatively short time, and research on their long-term insulation characteristics is still insufficient, particularly regarding the evolution of key electrical parameters such as dielectric loss under long-term operation. In this study, RIF bushing samples with typical internal insulation defects were designed and prepared. Under laboratory conditions, real operational environments were simulated by applying operational voltages for an extended period. The high-voltage dielectric loss changes before and after partial screen breakdown were systematically measured and analyzed. Additionally, a new equivalent circuit model for high-voltage dielectric loss was proposed, which effectively explains the transition process of dielectric loss with applied voltage and the phenomenon where the dielectric loss during voltage reduction is lower than that during voltage increase.