[Objective] This paper studied the biotransformation of the catalytic substrate dehydroepiandrosterone (DHEA) generated 7α,15α-diOH-DHEA by Gibberella intermedia C1 in the ionic liquid-water biphasic system. [Methods] The comparison of substrate conversion rates and product yields in five different ILs/water two-phase systems was carried out. And then, the optimization of bioconversion process in ionic liquid-water biphasic system was investigated to establish a suitable biphasic system with G. intermedia C1. [Results] The optimal conversion condition in ionic liquid-water biphasic system was as follows: 6 g/L DHEA and [Emim][EtSO4] (VILs:Vwater =0.8) were simultaneously added after 12 h fermentation of G. intermedia C1. In addition, the amplification experiments were carried out in 5 L fermentor. At 60 h fermentation of G. intermedia C1, the concentration of 7α,15α-diOH-DHEA increased to 5.03±0.21 g/L and the product molar yield reached 75.5%. [Conclusion] The optimal conversion condition in ionic liquid-water biphasic system was determined and the amplification experiments were carried out on the 5 L fermentor, which laid a solid foundation for industrial application further.