Abstract Most studies on frosting characteristics were conducted between −6 and 6 °C, which is the most common heat pump operation temperature region under frosting condition. But the cascade heat pump can operate at the temperature as low as −20 °C, thus, the frosting characteristics of low temperature environment should be studied. The frosting experiments are carried out in a climate chamber on a cascade type air-source heat pump. The outdoor temperature in the study varies from −18 °C to 6 °C and outdoor relative humidity varies from 70% to 90%. From the experimental results, it is found that the amount of frosting almost changes linearly with the frosting time, and the outdoor relative humidity has a great influence of the quantity. At low temperature conditions, frost still exists with lower the frosting rate. A new index called “frosting factor” is brought forth, which considers the moisture content difference as the frosting potential. Finally, it is found that the frosting factor is from 0.0389 to 0.05 g m−2 s−1 (g/kg)−1 and the average value is 0.0446 g m−2 s−1 (g/kg)−1, which can be utilized to estimate the frosting quantity under different outdoor environment conditions.