Defrosting　under　unnecessary　conditions　is　a　typical　and　undesirable　operating　phenomenon　for　air　source　heat　pump　(ASHP).　This　paper　aims　to　evaluate　the　ASHP　performance　under　unnecessary　defrosting　phenomena　in　different　regions　of　China.　Firstly,　energy　loss　for　two　kinds　of　unnecessary　defrosting　phenomena　was　quantified　by　60　days　field　test.　Secondly,　a　method　was　developed　to　predict　unnecessary　defrosting　times.　The　prediction　accuracy　can　be　controlled　within　10%.　Furthermore,　the　ASHP　performance　of　unnecessary　defrosting　was　evaluated　for　nine　typical　cities　in　China.　It　was　found　that　the　average　unnecessary　defrosting　rate　was　53%　for　northern　cities,　38%　for　mid-land　cities,　and　15%　for　southern　cities　during　a　heating　season.　Correspondingly,　the　total　energy　loss　was　233　MJkW-1,　161　MJkW-1,　and　44　MJkW-1.　These　results　indicate　that　unnecessary　defrosting　processes　should　not　have　been　ignored.　It　is　a　valuable　work　to　improve　the　defrosting　control　method.　(C)　2016　Elsevier　Ltd　and　IIR.　All　rights　reserved.