In order to study the frosting characteristics on the surface of the circular pipe below -60 ℃, a test rig for frosting visualization on double-pipe heat exchanger has been built based on cascade refrigeration system. Through a series of systematic experimental studies, it is found that periodic melting phenomenon occurs during the frost development and maturity period. First, the experimental phenomena of frost periodic progress are systematically summarized. Then, the melting criterion is added to the mathematical model of unsteady frosting on the surface of circular pipe by numerical method. The cycle of frosting and melting is simulated. The simulated results are similar to the experimental results, which shows that the model can accurately predict the melting process. The results show that the higher the humid air temperature is, the earlier the melting node appears; since the first melting phenomenon occurs, the interval of each melting phenomenon becomes longer.