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Quantum spin Hall (QSH) effect is promising for achieving dissipationless transport devices which can be achieved only at extremely low temperature presently. The research for new large-gap QSH insulators is critical for their realistic applications at room temperature. Based on first-principles calculations, we propose a QSH insulator with a sizable bulk gap as large as ∼0.22 eV in stanene film functionalized with the organic molecule ethynyl (SnC _2 H), whose topological electronic properties are highly tunable by the external strain. This large-gap is mainly due to the result of the strong spin–orbit coupling related to the p _xy orbitals at the Γ point of the honeycomb lattice, significantly different from that consisting of the p _z orbital as in free-standing group IV ones. The topological characteristic of SnC _2 H film is confirmed by the Z _2 topological order and an explicit demonstration of the topological helical Dirac type edge states. The SnC _2 H film on BN substrate is observed to support a nontrivial large-gap QSH, which harbors a Dirac cone lying within the band gap. Owing to their high structural stability, this two-dimensional large-gap QSH insulator is promising platforms for topological phenomena and new quantum devices operating at room temperature in spintronics.