A　general　distributed　model　with　a　non-steady-state　heat　exchanger　model　coupled　with　a　frost　model　was　developed　to　study　the　dynamic　behavior　of　an　airside　heat　exchanger　in　an　air-to-water　heat　pump　heater/chiller　unit.　The　effects　of　water　vapor　diffusion　and　uneven　fin　temperature　distribution　were　considered.　The　model　was　found　to　agree　well　with　reported　experimental　results.　Compared　with　the　routine　model,　the　present　model　has　higher　precision　of　frost　layer　thickness　especially　on　the　fin　surface.　Results　include　the　propagation　of　frost　formation　along　the　tube　and　its　effect　on　the　dynamic　characteristics　of　refrigerant,　air,　and　tube　sides.　According　to　the　results,　the　temperature　difference　between　air　and　tube　surface　temperature　was　proposed　to　be　the　main　driving　force　of　frosting.　Tube　surface　temperature　is　the　most　important　factor　affecting　frosting　when　there　is　little　variation　in　air　humidity.　Frost　at　the　fin　base　was　found　to　be　thicker　than　that　at　the　fin　tip　due　to　the　fact　that　the　frost　layer　grows　faster　with　lower　tube　surface　temperature.