This paper presents a novel air source heat pump for heating of buildings named air source heat pump with multiple parallel outdoor units (ASHPMO). Multiple outdoor units were connected in parallel with the aim of realising alternate defrosting and uninterrupted heating simultaneously. An experimental apparatus of the ASHPMO system was developed. The defrosting performance was experimentally investigated under different outdoor air temperatures, outdoor air relative humidity, and condensation temperatures, among other factors. The test results showed that the novel ASHPMO system could provide continuous heating when defrosting even under an outdoor air temperature of -10 degrees C. Variations in compressor vapour suction and discharge pressure and temperature were observed. The minimum heating capacity could still reach 60% of that without defrosting. Under the defrosting condition with outdoor air temperature -10 degrees C, both the heating coefficient of performance (heating COP) and total energy efficiency ratio (EER) of the system can reach to 2.0 and 2.32, respectively.