Abstract: To address this energy waste, this paper conducts a theoretical analysis by introducing relevant frost formation models and combines experimental data to derive a semiempirical formula for the "theoretically optimal defrosting amount". The formula mainly involves parameters such as cold surface temperature, moisture content of saturated water vapour, time, and wind speed. Moreover, this paper explains why defrosting does not occur during the early stage of frost formation and highlights that this factor has a relatively minor impact on the overall predictive performance. To verify the effectiveness of the formula, this paper conducted experiments on a manufacturer’s supplemental gas enthalpy booster-type secondary compression air-source heat pump under various operating conditions.Experimental tests were conducted in a temperature range of-4 ℃ to 4 ℃ at relative humidity levels of 65%, 70%, and 80%. The actual defrosting amount observed under different conditions was compared with the "theoretically optimal defrosting amount" calculated using a semiempirical formula, and it was found that the relative error between the two was relatively small, within ±10%. The study showed that the formula provided a relatively accurate prediction of the frost amount in the later period of frost formation, and it could serve as a reference for designing supplemental heating control and secondary compressor enthalpy for controlling the defrosting time of air-source heat pumps.