The aim of the present study was to optimize the solid-state fermentation with Aspergillus oryzae to degrade phytic acid and increase Aspergillus oryzae biomass growth in soybean meal, and to investigate the correlation between phytic degradation and fungal biomass growth. The response surface methodology by Box-Behnken design was used to optimize the ratio of substrate to water, inoculum size and duration. A total of 17 runs including 5 fermentation conditions and 5 replicates at the center point were set in this experiment. Each fermentation condition consisted of 3 levels (the ratios of substrate to water were 0.5, 1.0 and 1.5, inoculum sizes were 4%, 8% and 12%, and durations were 72, 96 and 120 h). Ergosterol was used as a tool to estimate Aspergillus oryzae biomass. The results showed that the minimum phytic acid content was 2.38 mg/g dry matter (DM) when the ratio of substrate to water was 0.5, inoculum size was 4% and fermentation duration was 120 h. The maximum ergosterol content was 396.74 mg/kg DM when the ratio of substrate to water was 0.5, inoculum size was 12% and duration was 120 h. In additoin, a significant negative correlation between phytic acid degradation and fungal biomass growth of substrate in solid-state fermentation was detected. In conclusion, the optimal conditions for phytic acid degration can be achieved when the ratio of substrate to water is 0.5, inoculum size is 4% and duration is 120 h, and the optimal conditions for Aspergillus oryzae growth can be achieved when the ratio of substrate to water is 0.5, inoculum size is 12% and duration is 120 h. Aspergillus oryzae is of high efficiency in phytic acid degradation, and shows a potential use in industry-scale solid-state fermentation. Ergosterol can be a good indicator for phytase generation or phytic acid degradation in solid-state fermentation of soyban meal.