Abstract: In order to improve the ability of Monacolin K produced by liquid fermentation of Monascus,this study used laboratory preservation Monacolin K output stable Monascus purpureus M1 as the test strain,intending to add different concentrations of citric acid in the process of cultivation of Monascus purpureus to analyze its effect on the synthesis of secondary metabolites of Monascus purpureus. The ultrastructure of the mycelium of Monacolin K in the experimental group and the control group was observed by scanning electron microscope. The expression of the key gene of Monacolin K was detected by fluorescence quantitative PCR,and the mechanism of increasing the production of Monacolin K was studied. The results showed that when citric acid concentration was 0.1%,the production of Monacolin K increased by 2.71 times than the original medium. The results of scanning electron microscope showed that the experimental group surface appeared more fold,thus speculated that citric acid might be through improving Monascus cell membrane permeability,timely secreted the Monacolin K synthesized by cells to the outside of cells,the cytoplasm Monacolin K concentration was reduced,and then increased the accumulation of extracellular Monacolin K.The expression of Monacolin K biosynthesis related key genes of Monacolin K in M.purpureus mycelium was determined by RT-qPCR determination,and it was found that citric acid could promote the expression of key genes(mokA~mokI and LaeA)of Monacolin K synthesis and thus improved the production of Monacolin K in M.purpureus. Therefore,it was concluded that the production of Monacolin K was the highest in the medium with citric acid concentration of about 0.1%. The possible mechanism was that citric acid changed the permeability of Monascus cell membrane and increased the expression of related genes.