The invention discloses a preparation method of ultra-pure sulfuric acid. In the preparation method, an industrial sulfur trioxide is used as a raw material, vaporized in a round vaporizer and condensed in the round vaporizer for secondary vaporization so as to obtain purified sulfur trioxide gas, and dilute sulphuric acid is used for circularly absorption to collect the target product of the ultra-pure sulfuric acid. The ultra-pure sulfuric acid obtained by the method has a main part content being above 96wt%, and the impurity content of metal ions conforms to a SEMI C12 standard. In addition, the purity of the sulfur trioxide is improved by adopting the secondary vaporization, and the purified sulfur trioxide is circularly absorbed with the dilute sulphuric acid of different concentrations, therefore, the detects of unstable product quality, slightly high metal ion impurity content in a product, large energy consumption, high cost and easy pollution in the traditional production method are overcome and the method is more suitable for large-scale continuous production.