It　has　been　known　that　the　actual　operating　performances　of　a　space　heating　ASHP　unit　are　significantly　impacted　by　the　joint　effect　of　ambient　air　temperature,　T-a,　and　relative　humidity,　RH.　To　enable　a　convenient　evaluation　of　the　operating　performances　of　a　space　heating　ASHP　unit　with　a　reasonable　accuracy,　an　equivalent　temperature　drop　(ETD)　method　has　been　developed.　The　ETD　method　transferred　the　impacts　of　frosting-defrosting　on　the　operating　performances　of　ASHP　units　into　that　of　an　equivalent　ambient　air　temperature　drop,　so　that　the　joint　effects　of　T-a　and　RH　on　the　actual　operating　performances　of　the　ASHP　units　may　be　assessed　using　a　single　parameter　of　equivalent　ambient　air　temperature.　In　this　paper,　the　models　for　predicting　the　output　heating　capacity　of　an　ASHP　unit　operated　at　both　frosting　and　non-frosting　conditions　were　firstly　established　through　either　regression　analysis　or　referring　to　previous　related　studies.　Secondly,　with　the　availability　of　the　models,　the　ETD　method　was　developed　for　evaluating　the　actual　operating　performances　of　ASHP　units　at　different　ambient　conditions.　Thirdly,　the　developed　ETD　method　was　verified　by　a　field　study　on　the　operating　performances　from　an　ASHP　unit　installed　in　Beijing.　The　field　study　results　demonstrated　that　for　any　frosting　operating　conditions,　using　the　ETD　method,　a　corresponding　non-frosting　condition　at　which　the　field　ASHP　would　output　the　same　heating　capacity　may　be　identified.　Furthermore,　the　use　of　ETD　method　can　also　distinguish　the　individual　impacts　from　either　T　a　or　RH　on　the　actual　operating　performances　of　an　ASHP　unit.　(C)　2020　Published　by　Elsevier　B.V.