In this study, acorn starch was investigated as a new material for fermenting production of citric acid by using a tannin tolerance mutant strain Aspergillus niger AA120. The mutant A. niger AA120 was obtained by initially atmospheric pressure plasma at room temperature (ARTP) mutagenesis and then tannin gradient domestication. ARTP experiments showed that a double-saddle shape of survival rate curve was achieved, and a positive mutation rate of 63.6% was reached by setting the implantation time of mutagenesis to 100s. In contrast to the original stain at the presence of 20.0g/L tannin in the medium, the selected mutant A. niger AA120 exhibits an increase of biomass by 43.76% to 32.9g/L, and citric acid production capacity by 20.34% to 130.8g/L, with 8% (w/w) of inoculation quantity, an initial pH of 6.2 and shaking speed of 250r/min. In this work, we present a referable method for the mutagenesis screening of the A. niger, and the application of acorn starch as a new raw material for the development of the citric acid industry.