Based on radiation characteristics of the material, the antenna temperature of the insulator contamination was measured with microwave radiometer. The main components of the contamination of insulator is salt and ash, and there is a complicated nonlinear relationship between the antenna temperature and equivalent salt density、equivalent ash density. In order to express the mathematical relationship more accurately,this paper proposes a LSSVM model optimized by the particle swarm optimization (PSO) to describe. The results show that the LSSVM model prediction accuracy is very high, according to the model can inverse the ESDD and NSDD with variable step iterative algorithm. The equivalent salt density of insulator contamination is a main basis to distinguish pollution level and draw the polluted area of power grid distribution. So it has very high practical significance for engineering application.