The single-layer (C4H9NH3)(2)PbBr4 hybrid perovskite has attracted enormous attentions because of the enhanced stability. However, the toxic lead in this single-layer material is a hard problem, which seriously restricts its application. Herein, the Pb atom is substituted by introducing Sn atom, and Br atom also is substituted by BH4 superatom to obtain the lead-free single-layer (C4H9NH3)(2)Sn(BH4)(4) hybrid halide perovskite. The calculated results by density functional theory (DFT) based first-principles calculations show that this single-layer lead-free hybrid halide perovskite has the lattice parameters of a = 8.38 angstrom and b = 8.25 angstrom, negative formation energy, and small cleavage energy, revealing that it is formed perovskite and can be stripped out from its bulk material by mechanical peeling or exfoliation method. Moreover, its direct band gap and effective electron (hole) masses can be tuned from 2.92 to 3.60 eV, and from 0.29 (0.26) to 0.72 (0.55) m(e) under the strain of -5% similar to 10%, respectively. Based on these results, we expect that the single-layer lead-free (C4H9NH3)(2)Sn(BH4)(4) hybrid halide perovskite is very useful for fabricating high-performance semiconductor devices.