Mixing-assisted oxidative desulfurization method of diesel fuel and the continuous desulfurization device thereof is disclosed herein. Because of the dangers of sulfur content in gasoline and other petroleum products, many governments have required the sulfur content of these petroleum products to be reduced so as to safeguard public safety and health. Desulfurization of fossil fuels is provided by the combination of fossil fuels with an aqueous mixture of ozone or hydrogen peroxide and a Tetraoctylphosphonium salt phase transfer catalyst, and the mixture is then reactive mixed to form oxidize sulfur compounds in the fuel. The polar oxidized sulfones species are removed via another mixing step. The desulfurization device (10) can be in the form of a portable device which provides for continuous mixing-assisted desulfurization for the selective removal of sulfur containing compounds from fossil fuels such as diesel fuel or other hydrocarbons.