Combined with method of ketoconazole resistance screening, a 7α,15α-diOH-DHEA high-producing mutant Colletotrichum lini ST-1 was obtained by compound mutation of NTG and low energy N+ ion beam implantation. With the substrate concentration of 10 g/L DHEA, the molar yield of 7α,15α-diOH-DHEA reached 34.2%, increased by 46.2% than that of the original strain. Then we optimized the medium. First, Plackett-Burman design was used to evaluate the effects of medium components on molar yield of the product. Results show that glucose, yeast extract and MgSO4·7H2O were the important parameters for the biotransformation process. Subsequently, the path of steepest ascent was used to approach the optimal levels. To obtain the optimal levels, central composite design and response surface analysis were carried out. The optimal medium was as follows (g/L): glucose 26.34, yeast extract 12.15, corn flour 3.00, FeSO4·7H2O 0.015, MgSO4·7H2O 0.14, KH2PO4 0.90. Under the optimal conditions, the molar yield of 7α,15α-diOH-DHEA reached 49.3%, which was 44.2% higher than that of using the medium before optimization.