The　dynamic　characteristics　of　an　air-to-water　heat　pump　under　the　frosting/defrosting　conditions　were　investigated.　The　experiments　were　conducted　on　a　heat　pump　with　the　nominal　heating　capacity　of　55　kW.　The　results　show　that　the　thermal　expansion　valve　(TEV)　started　to　have　the　intermittent　instability　or　hunting　in　a　feed-back　control　loop,　for　64　minutes　during　the　middle　of　the　frosting　period　due　to　the　airflow　maldistribution　at　the　face　of　finned　tube　heat　exchangers.　Moreover,　the　TEV　had　the　continuous　hunting　for　the　last　32　minutes　of　the　frosting　period.　The　combination　of　the　airflow　maldistribution　and　the　drop　in　airflow　rate　owing　to　the　frost　accumulation　resulted　in　the　flooding　of　the　evaporator　and　the　unceasing　hunting　of　the　heat　pump.　Furthermore,　a　critical　point　of　the　defrost　period　appeared,　after　which　the　system　parameters　increased　quickly　because　the　modes　of　heat　transfer　changed　outside　the　tubes　of　finned　tube　heat　exchangers.　Before　the　critical　point,　the　phase　change　heat　transfer　occurred　and　had　higher　heat　transfer　performance　outside　the　tubes　when　the　frost　was　melting.　But　after　the　critical　point,　the　air　natural　convection　occurred　and　had　lower　heat　transfer　performance　outside　the　tubes　because　the　frost　was　cleared　and　the　heat　exchanger　fans　were　turn　off.　Finally,　the　discharge　pressure　had　the　sudden　increase　dramatically　after　the　defrost　termination　or　during　the　start-up　of　the　frosting　period.　The　phenomenon　seemed　to　be　due　to　both　the　small　inner　volume　in　the　plate　heat　exchanger　and　larger　refrigerant　flow　rate　after　the　defrost　termination　or　during　the　start-up　of　the　frosting　cycle.