The system with the interplay between superconductivity, nontrivial band topology and strongly correlated electrons is of great interest due to the emergent novel quantum phases, such as topological superconductivity and strongly correlated topological phases. Such system is rarely found in realistic materials, recent examples are moiré graphene, ${\mathrm{FeTe}}_{0.55}{\mathrm{Se}}_{0.45}$ and kagome superconductors. In this work, we report that ${\mathrm{SrSn}}_3$ is a prototype of superconducting alloy with an odd $Z_2$ invariant, which is supported by the existence of topologically nontrivial surface states. Moreover, we find that strong spin-orbit coupling plays a dominant role in the formation of the flat-bands at Fermi level in ${\mathrm{SrSn}}_3$, which result in both considerable electron correlations (identified by a large carrier effective mass $m_C^{*}/m_0\approx 3.06$ and a large Kadowaki-Wood ratio $A/{\gamma }^2\sim 7.94\times {10}^{-4}\mu \mathrm{\Omega }\mathrm{cm}{\mathrm{mJ}}^2{\mathrm{mol}}^2{\mathrm{K}}^2$) and strong-coupling superconductivity ($2\mathrm{\Delta }/k_B{T}_c\sim 4.90$ and $T_c/T_F\sim 2.34\times {10}^{-3}$), that coexist with the topological surface states, characterized by the nontrivial Berry phase of de Hass-van Alphen oscillations (${\varphi }_B\sim \pi$). Remarkably, we observed a surface superconducting state, manifested as an extremely large surface-to-bulk critical field ratio ($H_{c3}/H_{c2}\sim 18.3$) and a linear temperature-dependent critical field. Our findings reveal that ${\mathrm{SrSn}}_3$ is a promising candidate for studying the interplay between nontrivial band topology and correlated electron phenomena induced by strong spin-orbit coupling.

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• Flat bands
• Magnetotransport
• Superconductivity
• Topological materials
• Transport phenomena
• First-principles calculations
• de Haas-van Alphen effect