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Recent studies have explored Continual Instruction Tuning (CIT) in Multimodal Large Language Models (MLLMs), with a primary focus on Task-incremental CIT, where MLLMs are required to continuously acquire new tasks. However, the more practical and challenging Domain-incremental CIT, focused on the continual adaptation of MLLMs to new domains, remains underexplored. In this paper, we propose a new Sparse Mixture of Expert (SMoE) based method for domain-incremental CIT in MLLMs. During training, we learn a domain-specific SMoE module for each new domain in every FFN sub-layer of MLLMs, preventing catastrophic forgetting caused by inter-domain conflicts. Moreover, we equip the SMoE module with a domain-specific autoregressive loss (DSAL), which is used to identify the most suitable SMoE module for processing each test instruction during inference. To further enhance the SMoE module’s ability to learn domain knowledge, we design an adaptive threshold-based router (AT-Router) that allocates computing resources (experts) to instruction tokens based on their importance. Finally, we establish a new benchmark to evaluate the efficacy of our method and advance future research. Extensive experiments show that our method consistently outperforms all competitive baselines.