The aim of this study was to evaluate the effects of adding soluble starch on the in vitro fermentation characteristics of degraded distilled grain, and to determine the optimal ratio of structural carbohydrate (SC) to nonstructural carbohydrate (NSC) for in vitro fermentation of degraded distilled grain. The distilled grain was degraded by Phanerochaete chrysosporium for 20 days to reduce the content of lignin. The ratios of SC/NSC were designed as 1.66, 2.06, 2.46, 2.86, 3.26 and 3.66 by adding different amounts of soluble starch into each group of in vitro fermentation. Each group had 5 replicates. The results showed that after degrading by Phanerochaete chrysosporium, the acid detergent lignin (ADL) of distilled grain was reduced by 16.4% (P<0.01) and the content of crude protein was increased by 6.04% (P<0.01). In the experiment of in vitro fermentation, the cumulative gas production (GP) was significantly increased (P<0.01) with the levels of soluble starch increasing. Adding soluble starch to change the SC/NSC ratio could significantly affect the microbial protein (MCP), total volatile fatty acid (TVFA), and ammonia nitrogen concentrations (P<0.01). Through regression analysis of SC/NSC ratio (x) and MCP (y) (y=―0.826x2+3.947x+1.189), the optimal SC/NSC ratio was 2.39. Adding soluble starch could significantly increase the propionate/TVFA ratio (P<0.05), but reduce the acetate/propionate ratio (P<0.05). These results indicated that adding soluble starch could improve the digestibility of degraded distilled grain, and the optimal SC/NSC ratio for in vitro fermentation was 2.39.