Defrosting　performance　evaluation　is　an　important　aspect　for　pre-delivery　inspection　of　air　source　heat　pumps　(ASHPs).　According　to　GB/T　25127-2020　and　ANSI/AHRI　550/590-2023,　defrosting　duration　ratio　(alpha)　alpha　)　should　not　exceed　20　%　of　the　frosting-defrosting　cycle.　This　value　has　remained　unchanged　for　over　20　years.　However,　with　the　rapid　iteration　of　defrosting　technology,　the　alpha　of　new　ASHPs　has　been　decreased　significantly.　Hence,　the　applicability　of　this　index　value　deserves　to　be　deliberated.　To　solve　this　issue,　a　method　for　determining　the　optimal　alpha　was　proposed　firstly.　Then,　the　alpha　under　different　characteristic　index　(　CICO　)　values　at　2/1　degrees　C　and　2/0　degrees　C　were　investigated　at　the　optimal　and　original　defrosting　initiating　time　point　(　t　opt　,　t　ori　),　using　twelve　ASHPs　from　nine　manufacturers.　Results　indicates　the　frosting　duration　(tf)　t　f　)　is　directly　proportional　to　defrosting　duration　(tdf),　t　df　),　at　the　same　ambient　air　condition　and　CICO.　.　For　Unit　C,　when　t　f　is　increased　from　30.0　min　to　60.0　min　and　90.0　min　at　the　CICO　of　10.0　x　106,　6　,　t　df　increases　from　2.43　min　to　3.20　min　and　4.03　min　at　2/1　degrees　C,　respectively.　No　matter　at　t　opt　nor　t　ori　,　the　range　of　alpha　is　1.4%-9.9　%.　It　is　suggested　that　the　10　%　is　used　as　the　new　index　for　defrosting　performance　evaluation　of　ASHP.