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Background: Allergen-specific immunotherapy, including subcutaneous immunotherapy (SCIT) and sublingual immunotherapy (SLIT), improves the disease progression of allergic rhinitis (AR). SCIT and SLIT exhibit similar efficacy, but SLIT has less systemic reactions. However, few studies have investigated the underlying mechanisms of SLIT treatment. In this study, we explored the efficacy of SLIT under different treatment durations and immunological changes. Methods: This retrospective study was conducted from August 2017 to August 2022 in our hospital. A total of 314 children who underwent SLIT were divided into the following groups based on their treatment duration: the 1 year group (6 months–1 year), the 2 years group (1–2 years), and the 3 years group (2–3 years). The treatment efficacy was confirmed using a combined symptom and medication score (SMS). Multiple serum cytokines were measured using Luminex. Various immune cells in PBMCs were determined using flow cytometry. Results: The total nasal symptom score (TNSS), rescue medication score (RMS), and SMS of the 3 years group was significantly different from those of the 1 years and 2 years groups. At the end of the 2 years following cessation of SLIT, the following results were observed in the 3 years group: 1) the TNSS, RMS, and SMS had significantly improved, 2) the serum IL-10, TGF-beta, and IL-35 levels had increased significantly, and 3) the percentages of regulatory T cell, regulatory B cell, and follicular regulatory T cell increased significantly. Conclusion: Our results suggest that 3 years of SLIT is necessary for long-term effects and continued immunological changes.