SnSe2　is　an　anisotropic　binary-layered　material　with　rich　physics,　which　could　see　it　used　for　a　variety　of　potential　applications.　Here,　we　investigate　the　gas-sensing　properties　of　SnSe2　using　first-principles　calculations　and　verify　predictions　using　a　gas　sensor　made　of　few-layer　SnSe2　grown　by　chemical　vapor　deposition.　Theoretical　simulations　indicate　that　electrons　transfer　from　SnSe2　to　NO2,　whereas　the　direction　of　charge　transfer　is　the　opposite　for　NH3.　Notably,　a　flat　molecular　band　appears　around　the　Fermi　energy　after　NO2　adsorption　and　the　induced　molecular　band　is　close　to　the　conduction　band　minimum.　Moreover,　compared　with　NH3,　NO2　molecules　adsorbed　on　SnSe2　have　a　lower　adsorption　energy　and　a　higher　charge　transfer　value.　The　dynamic-sensing　responses　of　SnSe2　sensors　confirm　the　theoretical　predictions.　The　good　match　between　the　theoretical　prediction　and　experimental　demonstration　suggests　that　the　underlying　sensing　mechanism　is　related　to　the　charge　transfer　and　induced　flat　band.　Our　results　provide　a　guideline　for　designing　high-performance　gas　sensors　based　on　SnSe2.