The icing of transmission lines seriously threatens the safe of power grid. The icing process of transmission lines is complicated and there are many influences. The accurate understanding of the law of icing is the basis for the establishment and optimization of a icing numerical model. Based on natural icing tests, the tests of different kinds of conductor are carried out on Xuefeng mountain natural icing test base, and the influences of conductor diameter, conductor surface treatment, icing type and torsion on icing process are studied. The characteristics of icing on conductors with different diameters are analyzed by calculating the water droplet collision efficiency. The results show that under the natural environment, the wind velocity is the key factor which determines the rime shape on transmission lines. Meanwhile, the icing thickness increases nonlinearly with time, the smaller the conductor diameter, the faster the icing thickness increases. The conductor torsion makes the leeward side of conductor changes to the windward side, which increases the icing rate. When the icing type becomes glaze, in addition to the windward wing ice, there are more likely icicles freezing under a conductor, as a result, the ice structure becomes more complex.