Using thermal balance equation during the process of hail melting, the approximate relationship of 0℃ layer height (h0) and hail melting ability was obtained, and the influence of h0 to zero layer hail radius (r0) and ground hail radius (rg) was analyzed under the condition of small scale hail. The results show that rg changes with h0 and r0, so the hail cloud index setting such as radar echo height and center intensity is related to the change of h0, when h0 changes in the range of 2000-6000 m, the threshold of minimum zero layer hail radius changes between 0.32 cm to 1.08 cm, and the threshold of minimum updraft velocity changes from 11.5 m·s-1 to 21.2 m·s-1. The results show a general understanding on the change of 0℃ layer height to hail melting, which can provide a reference for hail forecast and hailsuppression operation, and reduce the false alarm rate of WSR88D enhanced HDA algorithm in hail size predication.