Extremely pure naphthalene is the raw materials of some high level industrial products, and it can be attained from the industrial naphthalene fraction by desulphurization and purification. The primary impurity in industrial naphthalene fraction is the sulfide thionaphthene, which is very difficult to be removed off by ordinary separation methods. Oxidative desulphurization with oxidant hydrogen peroxide and catalyst formic acid was studied, and the effects of formic acid amount, hydrogen peroxide amount, reaction temperature and reaction time on desulphurization ratio were investigated experimentally. The result indicates that the desulphurization rate simply increases with the increasing of formic acid amount, but it increases at first and then goes down with the increasing of hydrogen peroxide amount, and there is an optical amount range. The viscosity decreases with the increasing of temperature, which is help to the mixing of oil and water phase, and thus to improve the reaction and the desulphurization rate, and the optical temperature is 89~92 ℃. Under the optimized condition of n(H)/n(F)=3.65, n(H)/n(S)=8.49, 89 ℃ and 60 min, the desulphurization rate can reach to 97.90%, and after purified the oil phase by distillation to remove the [KG*4]resultant [KG*4]and [KG*4]the non-sulfide impurity, the  content  of  naphthalene  in  the  product is  more  than 99.90%, and is clearly higher than the industrial product(97.06%).