Under the internal pressure during operation, the concrete lining and surrounding rock of a lined rock cavern are generally in a plastic state. To accurately reveal the load transfer mechanism of the cavern structure under elastoplastic conditions, a numerical model of the composite structure comprising the sealing layer, lining, and surrounding rock is established based on elastoplastic theory. A corresponding method for calculating the internal pressure load-sharing ratio is proposed, and the influence patterns of multiple key factors are systematically investigated. The results indicate fundamental differences between the elastoplastic and elastic solutions. When material plasticity is considered, the load-sharing characteristics of the sealing layer change significantly: the load-sharing ratio of the steel-lined sealing layer increases notably, while that of the concrete lining decreases substantially. The surrounding rock remains the primary structure bearing the internal pressure. Sensitivity analysis further reveals that the stiffness of the sealing layer material, the grade of the surrounding rock, and the reinforcement ratio of the lining are key factors dominating the load distribution. Specifically, a flexible sealing layer is more conducive to realizing the design philosophy of load-bearing by the surrounding rock. A reduction in the grade of the surrounding rock significantly affects the load-sharing ratio, leading to a larger proportion of the load being carried by the lining. Increasing the reinforcement ratio of the lining effectively enhances its hoop restraint and load-bearing capacity, resulting in a notable increase in its share of the internal pressure load. In contrast, factors such as the structural joint type of the lining, variations in lining thickness, and fluctuations in operating pressure within a certain range have relatively limited impact on the overall load distribution.