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.