An　experimental　study　on　the　frosting　process　in　a　confined　space　through　a　horizontal　tube　frosting　visualization　test　bed　is　conducted　to　optimize　the　structure　of　the　heat　exchanger　and　improve　its　defrosting　performance.　The　growth　rule　of　frost　layer　thickness　along　the　length　of　a　tube,　the　variations　of　frost　accumulation　and　heat　transfer　rate　with　time　are　evaluated.　The　influences　of　wet　air　initial　flow　rate,　temperature,　relative　humidity　and　wall　temperature　on　the　frost　accumulation　and　heat　transfer　rate　on　the　surface　of　the　tube　are　analyzed.　Experimental　results　show　that　the　thickness　of　the　frost　layer　gradually　increases　along　the　length　of　the　tube　at　the　beginning　of　frosting.　Then,　the　frost　layers　are　the　thickest　in　the　middle　of　the　round　tube,　and　the　lowest　near　the　inlet,　respectively.　Frosting　begins　at　the　outlet　of　the　air　flow　passage.　The　frost　accumulation　shows　an　increasing　trend　with　time　passing,　while　the　heat　transfer　rate　increases　first　and　then　decreases.　Higher　wet　air　flow　rate,　wet　air　temperature　and　relative　humidity　and　lower　wall　temperature　correspond　to　larger　amount　of　frosting　and　heat　transfer　rate,　among　which　the　wall　temperature　has　less　influence　on　the　frost　accumulation　and　heat　transfer　rate.　The　resulting　characteristics　of　frosting　and　heat　transfer　rate　have　certain　reference　value　for　heat　exchanger　design.　©　2020,　Editorial　Office　of　Journal　of　Xi＇an　Jiaotong　University.　All　right　reserved.