Abstract: The flashover of silicone rubber outer insulation of DC wall bushings is one of the key insulation issues that endanger the UHV DC transmission project. In order to improve the flashover performance of silicone rubber, two kinds of liquid silicone rubber (LSR), which can be used for external insulation of high voltage wall bushings, were fluorinated with 12.5% fluorine/nitrogen mixture in laboratory. The DC flashover performance, surface physicochemical characteristics and basic electrical properties of two kinds of LSR sample before and after fluorination were comparatively studied. Flashover tests using a sheet sample/finger electrode configuration show that the fluorination can significantly increase the DC flashover voltage of two kinds of LSR sample, but with different increase rates (51.7% and 31.0%). ATR-IR and XPS analyses indicate that silica contents in two kinds of LSR are different, the fluorinated surface layers contain not only C—F bonds but also a considerable amount of Si—F bonds, and fluorine atoms are introduced into the surface layer by substituting hydrogen of methyl groups or methyl groups or by reacting with silica. Surface potential decay measurements and conductivity measurements show that two kinds of LSR sample have different bulk conduction and different surface potential decay characteristics. The increase in surface conduction or the suppression of surface charge accumulation caused by fluorination is the main reasons for the increase in DC flashover voltage.