Adopting thermal energy storage (TES) based reverse cycle defrosting (RCD) for cascade air source heat pumps (ASHPs) is a feasible way to tackle the defrosting problem when the cascade ASHP is operated in heating process. However, during TES-based RCD, the phase change material (PCM) undergoes multi-mode heat discharging process, resulting from different operating conditions in high temperature cycle (HTC) and low temperature cycle (LTC) of a cascade ASHP unit. The heat provided to HTC and LTC is therefore different, thus influences the performances of space heating and defrosting. In this paper, the heat coupling relations between the HTC and LTC during TES-based RCD were experimentally investigated. Five cases with the same amount of frost under different outdoor conditions were conducted. The total heat output to indoor space in HTC, the total heat consumption in LTC for defrosting and the heat provided to HTC and LTC by the PCM based heat exchanger (PCM-HE) were reported, and a discussion on the factors that influence heat coupling relations was then given. Contributions of this study are supposed to be used for adjusting the control strategy for TES-based RCD of a cascade ASHP unit. (C) 2019 Published by Elsevier Ltd.