A titanium sub-oxide/ruthenium oxide composite electrode and a preparation method and application thereof. Titanium-based titanium sub-oxide nanotubes is taken as a bottom layer, and titanium sub-oxide doped ruthenium oxide is taken as a surface composite active layer. A titanium substrate is anodized in a fluorine-containing ionic electrolyte, taken out, subjected to heating and roasting, cooled and then subjected to cathodic electrochemical reduction in polarizing liquid, so that a titanium-based titanium sub-oxide nanotube electrode is obtained; and then the titanium-based titanium sub-oxide nanotube electrode is taken as a cathode to be electrodeposited in a ruthenium trichloride electrolyte doped with titanium sub-oxide powder, taken out and then subjected to heating and roasting, so that the titanium sub-oxide/ruthenium oxide composite electrode is obtained.