Current production of diosgenin mainly depends on the acid hydrolysis of steroidal saponins from Dioscorea plants, and, in China especially, Dioscorea zingiberensis C. H. Wright (DZW) is used. The experimental results we obtained demonstrated that furostanol saponins, as the main constituents in DZW, were prone to generate 25-spirosta-3,5-diene as side product during the acid hydrolysis process, while spirostanol saponins hardly generated 25-spirosta-3,5-diene. This 25-spirosta-3,5-diene was the key reason leading to the low yield of diosgenin from DZW by acid hydrolysis. Effective conversion of furostanol saponins into spirostanol saponins can avoid the generation of 25-spirosta-3,5-diene so as to increase the yield of diosgenin, suggesting the importance of this preliminary conversion for improving the yield of diosgenin from DZW by acid hydrolysis. The conversion of furostanol saponins into spirostanol saponins can be performed by either enzymatic hydrolysis or spontaneous fermentation, whereas enzymatic hydrolysis is more controllable compared with spontaneous fermentation.