The original strain of Aspergillus oryzae CS3.03 isolated from koji powder was treated with the composite mutagen UV-DES. The mutants were screened with the method of casein plate and agar block. According to re-screening results detecting with the method of Folin-phenol, and the better screening means was found. At the same time, the fermentation ability of high protease activity mutants were tested in the state of low-salt solid-state fermentation technology, and the genetic stability of the best was tested by repeated multiplying. The results showed that the screening efficiency of agar block method was significantly higher than casein plate method. Five mutants of high protease activity with production potential were obtained by re-screening. The result of soy sauce fermentation test proved that the content of ammonia nitrogen and the utilization ratio of total nitrogen were higher than those of the original strain. The ammonia nitrogen in soy sauce fermented by mutant P7 and Y5 increased 14.56% and 11.26% respectively, and the utilization ratio of total nitrogen increased 2.92% and 3.12% in the same situation. Genetic stability test showed that P7 had great industrial application value and genetic stability, and its average protease activity was 4 994.18 U/gdb, which was 173.92% higher than that of the original bacteria.