To tackle the problem of frosting occurred on the outdoor coil surface when an air source heat pump (ASHP) is operated in winter, a novel frost-free ASHP system has been developed. However, the dehumidification performance and the frost-free time of the system operated are highly dependent on the electronic expansion valve (EEV) opening of the 1st stage throttle. In addition, the coefficient of performance (COP) of the systems is controlled by the EEV opening of the 2nd stage throttle. Therefore, the objective of this study is to investigate the effect of the EEV opening on the system thermodynamic performance in terms of dehumidification efficiency, regeneration efficiency, compression ratio, heating capacity and COP. It was observed that the optimal performance was achieved when the EEV openings of the 1st stage throttle and 2nd stage throttle were 75% and 40% respectively. In addition, a correlation of COP with the EEV openings of the 1st stage throttle and 2nd stage throttle was developed to optimize the system performance. The results showed that the system thermodynamic characteristics were highly sensitive to the variation in the EEV openings of both the1st stage throttle and 2nd stage throttle.