Aqueous Zn//MnO₂ rechargeable zinc-ion batteries (ZIBs) possess potential applications in electrochemical energy storage due to their safety, low cost, and environmental friendliness. However, manganese dioxide as the cathode material has poor cycle stability and low conductivity. In this work, the SnO₂@K-MnO₂ (SMO) composite was prepared using the hydrothermal method followed by the treatment with SnCl₂ sensitization, and its electrochemical characteristics were examined using SMO as the cathode material for ZIBs. The reversible specific capacity reaches 298.2 mA·h·g⁻¹ at 0.5 A·g⁻¹, and an excellent capacity retention of 86% is realized after 200 cycles, together with a high discharge capacity of 105 mA·h·g⁻¹ at 10 A·g⁻¹ and a long-term cycling life of over 8000 cycles with no apparent capacity fade. This cathode exhibits a long cycle life up to 2000 cycles at 2 A·g⁻¹ with the mass loading of 5 mg·cm⁻², and the battery maintains the capacity of 80%. The reversible co-embedding mechanism of H⁺/Zn²⁺ in such a Zn//SMO battery was confirmed by XRD and SEM during the charge/discharge process. This work can enlighten and promote the development of advanced cathode materials for ZIBs.