A topological insulator is a novel quantum state, characterized by symmetry-protected non-trivial edge/surface states. Our first-principle simulations show the significant effects of the chemical decoration on edge states of topological Bi(111) bilayer nanoribbon, which remove the trivial edge state and recover the Dirac linear dispersion of topological edge state. By comparing the edge states with and without chemical decoration, the Bi(111) bilayer nanoribbon offers a simple system for assessing conductance fluctuation of edge states. The chemical decoration can also modify the penetration depth and the spin texture of edge states. A low-energy effective model is proposed to explain the distinctive spin texture of Bi(111) bilayer nanoribbon, which breaks the spin-momentum orthogonality along the armchair edge.PACS numbers: 71.15. Mb, 73.43.Nq, 73.20.Fz, As an insulating state with symmetry-protected gapless interface electronic modes, the topological insulator (TI) has received considerable attention recently [1][2][3]. The edge conduction channels of two-dimensional (2d) TI exhibits quantum spin Hall effect within bulk gap [4]. A single bilayer Bi(111) film has been predicted to be a 2d TI with a large band gap of about 0.5 eV [5][6][7], while other 2d TIs, such as HgTe/CdTe quantum wells [8] and InAs/GaSb quantum wells [9], have gaps of only several tens of meV at best. Recently, Bi(111) bilayer has been readily grown on Bi 2 Te 3 or Bi 2 Se 3 substrates [10][11][12][13][14][15]. Therefore, it is very promising for room-temperature TI-based devices. However, the native edges of Bi bilayer suffer from the simultaneous presence of both trivial and non-trivial edge modes [6,12], which complicate the fundamental investigation of its topological properties and eventual applications. Similar complication had perplexed the interpretation of surface states in threedimensional (3d) TI Bi 1−x Sb x [16,17]. Although localization in Anderson's sense will suppress trivial conducting channels, quantitively localizing trivial channels will still be an experimental challenge. A wide distribution of conductance induced by multiple edge states is not desirable for accurate transport measurement [6,18,19].The complicated edge or surface states may be a generic problem associated with dangling bond states at the termination of 2d or 3d TIs. In this Letter, we report a first-principle analysis of chemical decoration of the edge states of Bi(111) bilayer, which, as we show, is an effective route for precise engineering of conducting edge states. We demonstrate that chemical passivation quantitatively removes the trivial edge bands in Bi bilayer nanoribbons, restoring the desired Dirac dispersion of the non-trivial edges. We further compute transport and optical signatures of the chemical decoration of the Bi bilayer edges, which can be assessed experimentally. In particular, we suggest that the Bi bilayer nanoribbons, with and without chemical passivation, offer a simple system for assessing conductance fluctuation of edge states...