Potassium-ion batteries, due to their low cost and other advantages, are considered a complementary technology to lithium-ion batteries in the field of large-scale energy storage. In recent years, they have received widespread attention from researchers. Bismuth sulfide has been identified as a promising anode for potassium-ion batteries due to its high theoretical specific capacity and low cost. Nevertheless, the polysulfide shuttle and severe volumetric expansion severely plague its commercial application on a large scale. Herein, using SnCl₄ as the Sn source and employing a mixed surfactant of oleylamine/oleic acid with significant spatial steric hindrance, ultra-small Sn-doped Bi₂S₃ (Bi_1.9Sn_0.1S₃) nanorods were first prepared under hydrothermal conditions. Next, through the polymerization reaction of dopamine, a coating layer of approximately 5 nm thickness was formed on the surface of the nanorods. Finally, by ordered electrostatic self-assembly, PDDA-modified nanorods were combined with MXene to obtain PDA-coated Sn-doped Bi₂S₃/MXene (BSPM-20) composites for alkali metal-ion storage. The obtained nanocomposite displays a considerable reversible capacity of 599 mAh g⁻¹ at 0.1 A g⁻¹ and impressive cycling stability over 1000 cycles with a capacity decay of 0.007% per cycle. It also delivers a high reversible capacity of 756 mAh g⁻¹ for Li storage and 650 mAh g⁻¹ for Na storage at 0.1 A g⁻¹, respectively. In addition, we have revealed the mechanism of the dual confinement effect of PDA and MXene on Bi_1.9Sn_0.1S₃ through experiments and DFT calculations. At the same time, it has also been clarified through structural characterization and kinetic analysis that oleic acid/oleylamine, as a capping agent, reduces the scale of Bi_1.9Sn_0.1S₃ by inhibiting grain growth, thereby improving the kinetic rate of potassium storage reaction. This work provides ideas for solving the issues of long-term cycles of metal sulfide.