This　paper　presents　an　experimental　investigation　on　the　behavior　of　an　air-source　heat　pump　operating　under　frosting/defrosting　conditions.　Dynamic　data　of　parameters,　such　as　the　frost　layer　thickness,　compressor　suction/discharge　pressures,　fin　temperature,　heating　capacity,　power　consumption,　and　fan　electric　current,　were　acquired　during　the　test.　Based　on　these　experimental　data,　the　operating　characteristics　of　the　heat　pump　under　frosting/defrosting　conditions　were　analysed,　and　the　influence　of　frosting/defrosting　on　the　performance　and　reliability　of　the　heat　pump　was　investigated.　It　was　shown　that　a　critical　frost　layer　thickness　existed,　below　which　the　heat　pump　presented　no　noticeable　parameter　changes.　When　the　frost　layer　grew　above　the　critical　value,　the　compressor　discharge　pressure　increased　rapidly　with　a　slight　decrease　in　the　suction　pressure,　but　the　shaft　power　remained　almost　unchanged.　However,　due　to　the　increase　in　airflow　resistance,　the　airflow　rate　dropped　and　the　fan　electric　current　increased　abruptly,　leading　to　fan　performance　deterioration　and　a　reduction　in　the　system　heating　capacity.　At　50　per　cent　nominal　airflow　rate,　the　frost　layer　formed　later　and　grew　slower　than　at　the　nominal　airflow　rate.　Under　defrosting　conditions,　the　suction　pressure　experienced　a　large　drop,　which　led　to　frequent　on-off　switching　of　the　compressor　due　to　pressures　falling　beneath　the　low-level　limit.　This　could　be　avoided　by　recirculating　a　portion　of　the　high　pressure　gas　in　the　compressor　receiver　back　to　the　compressor　inlet　through　a　pipe　fitted　with　a　solenoid　valve　controller.