The invention provides a production method of an ultra low carbon steel. The method comprises the steps of: employing a blowing process for desulfurization and slag removal of molten iron, with control of oxygen blowing time and smelting cycle of a converter, argon bottom-blowing in the whole course, and control of S content in slag lime, [C] content at converter end point, and temperature and oxygen activity at the endpoint, adding lime in a tapping process, and modifying ladle top slag after tapping; employing natural decarburization to control carbon content in an RH, wherein Als in molten steel is less than 0.03% after deoxidation, carrying out vacuum-breaking, adding aluminum powder and top slag modifier into the top slag, blowing argon with stirring, vacuumizing again, and carrying out clean circulation for 2-10 min; and carrying out continuous casting with protective casting in the whole course, wherein a nitrogen increase amount in the molten steel is controlled at less than 2 ppm. The invention can effectively reduce temperature drop in a transmission process, reduce oxygen blowing heating rate in a subsequent RH refining treatment, save energy and shorten the time of refining. Besides, the method can effectively guarantee modification effect of the top slag, prevent nozzle clogging and realize sequence casting. Therefore, the method is particularly applicable to the production of high purity ultra low carbon steel.