Oat bran was used as a substrate, and red yeast rice powder was added to provide fermented bacteria for mixed fermentation. The effective lipid-lowering ingredients in the mixture were used as evaluation index, and the fermentation process conditions (fermentation time, fermentation water pH, and amount of red yeast rice powder) were optimized by a single factor experiment. The results showed that the optimal fermentation time was 72 h, the pH of the fermentation water was 6.7, and the amount of added Monascus powder was 10 g. Under this condition, the contents of monacolin K, Monascus pigment, and β-glucan in the fermentation product were 3.76 mg/g, 27.58, and 3.31%, respectively. Compared with the raw mixture of oat bran and red yeast rice powder, monacolin K and Monascus pigments increased by 38.2% and 23.0% respectively, while β-glucan only decreased by 41.2%. The fermentation product contains three kinds of high-fat lipid-lowering ingredients at the same time, and it is meaningful for the comprehensive utilization of oat bran and the development of lipid-lowering functional foods.