To　quantify　the　performance　drop　of　the　air　source　heat　pump　(ASHP)　system　under　a　special　kind　of　mal-defrost　phenomenon　appearing　in　moderate　climate　conditions,　a　field　test　was　conducted　for　8　days　at　the　initial　stage　of　a　heating　season　in　Beijing,　China.　The　mal-defrost　was　found　with　the　more　than　60%　frosted　area　of　the　outdoor　heat　exchanger　after　the　system　running　5　days.　During　this　frosting　period,　the　system　COP　was　significantly　degraded,　only　2.3　under　an　environment　temperature　of　7.9　degrees　C.　Comparing　the　test　data　before　and　after　frosting,　it　was　found　that　the　mal-defrost　decreased　the　COP　up　to　40.4%　and　the　heating　capacity　to　43.4%.　Such　low　energy　efficiency　continued　quite　a　long　time　until　the　defrost　control　was　started　up　manually　by　the　authors.　After　defrosting,　the　COP　reclaimed　to　the　normal　level　of　5.0.　The　origins　of　this　special　mal-defrost　phenomenon　were　discussed.　And　some　suggestions　were　proposed　to　modify　the　current　defrosting　control　strategy,　which　were　helpful　to　avoid　the　mal-defrost　problem　in　the　current　ASHP　system　and　therefore　improve　the　system　performances.　(C)　2012　Elsevier　Ltd.　All　rights　reserved.