Researching on ESDD safe range is the important work of determining maintenance cycle of insulators. Through theoretical analysis and calculation and ANSYS thermal simulation and experiments, including analysis and calculation of alpine and high-polluting coal field railway insulators dirty components, leakage current, the heat flow field, water collection, water evaporation, eatl,It analyzed the surface water balance and heat balance of insulators of the alpine and high-polluting coal field of railway, Not only explained the reasons of the phenomenon that in this environment the insulator surface isn’t frozen, but also identified the safe range of online monitoring ESDD in which the insulator isn’t frozen and does not meet flashover and ice flash .