Two-dimensional　chalcogenides　could　play　an　important　role　in　solving　the　short　channel　effect　and　extending　Moore＇s　law　in　the　post-Moore　era　due　to　their　excellent　performances　in　the　spintronics　and　optoelectronics　fields.　In　this　paper,　based　on　theoretical　calculations　combining　density　functional　theory　and　non-equilibrium　Green＇s　function,　we　have　systematically　explored　the　intrinsic　mobility　in　the　Ga2SSe　monolayer　and　quantum　transport　properties　of　sub-10　nm　Ga2SSe　field-effect　transistors　(FET).　Interestingly,　the　Ga2SSe　monolayer　presents　high　intrinsic　electron　mobility　up　to　10(4)　cm(2)　(V　s)(-1).　It　is　highlighted　that　the　intrinsic　mobility　in　the　Ga2SSe　monolayer　is　significantly　restrained　by　phonon　scattering,　where　the　out-of-plane　acoustic　mode　and　high-frequency　optic　phonon　mode　are　found　predominantly　coupled　with　the　electrons.　As　a　result,　the　n-type　doping　sub-10　nm　Ga2SSe　FETs　represent　distinguished　transport　properties.　In　particular,　even　the　gate　length　is　shortened　to　3　nm,　the　on-state　current,　delay　time　and　power　consumption　of　the　n-type　doping　Ga2SSe　FET　along　the　armchair　direction　can　reach　the　International　Technology　Roadmap　for　Semiconductor　industry　standards　for　high-performance　requirements.　Our　present　study　paves　the　way　for　the　application　of　Ga2SSe　monolayers　in　ultra-small　sized　FETs　in　the　post-silicon　era.