Although large language models (LLMs) like ChatGPT have demonstrated considerable capabilities in general domains, they often lack proficiency in specialized fields. Enhancing a model’s performance in a specific domain, such as law, while maintaining low costs, has been a significant challenge. Existing methods, such as fine-tuning or building mixture of experts (MoE) models, often struggle to balance model parameters, training costs, and domain-specific performance. Inspired by composition to augment language models, we have developed Law-Neo, a novel model designed to enhance legal LLMs. This model significantly improves the model’s legal domain expertise at minimal training costs, while retaining the logical capabilities of a large-scale anchor model. Our Law-Neo model outperformed other models in comprehensive experiments on multiple legal task benchmarks, demonstrating the effectiveness of this approach.