Lightning protection lines have a higher risk of breakage due to icing compared to transmission lines, and existing methods are difficult to effectively identify the process of lightning protection line melting. Based on the real-time tensile force monitoring data of 200 lightning protection lines during the melting ice process, this paper analyzed the ice weight change rate of per unit length and the ice weight change probability in the melting ice instant for lightning protection lines with different voltage levels of 110 kV, 220 kV, and 500 kV. Then, a calculation model of ice weight for per unit length was established by considering about the features of the horizontal stress, vertical stall spacing, and vertical loading change during the ice melting process, and the calculation accuracy was verified based on actual case. Results show that the ice weight change rate threshold of lightning protection line with different voltage levels are −0.035 kg/10 min, −0.049 kg/10 min, −0.072 kg/10 min, corresponding to the probability of 89.26%, 88.04%, 90.13%. The ice weight change thresholds of lightning protection line with different voltage levels are 6.51 kg, 8.22 kg and 10.26 kg, with probabilities of 91.16%, 89.72% and 88.17%, respectively. The study results can provide effective guidance for ice melting process and the anti-ice engineering design of the lightning protection line.