The invention provides a siraitia grosvenorii concentrated juice preparation method, which comprises: A) sterilizing siraitia grosvenorii, crushing, extracting, and carrying out solid-liquid separation to obtain an extract liquid; b) mixing the leach liquor, a color fixative, cation exchange resin and an acidophilic leavening agent for fermentation, and performing solid-liquid separation to obtain acidophilic fermentation liquor; c) adjusting the pH value of the acidophilic fermentation liquor to be alkaline, adding silica gel and a basophilic leavening agent for fermentation, and performing solid-liquid separation to obtain basophilic fermentation liquor; d) adjusting the pH value of the basophilic fermentation liquor to be acidic, passing through a nanofiltration membrane, and collecting trapped fluid; and concentrating and sterilizing the trapped fluid to obtain the momordica grosvenori concentrated juice. According to the method, the energy value can be reduced at the same time, the green taste similar to that of green grass is eliminated, and certain aromatic flavor substances are contained; meanwhile, the method does not need an independent decolorizing procedure, browning can be effectively inhibited in the concentrated juice fermentation process, and the browning substrate is greatly reduced in the fermentation process, so that the concentrated juice is not prone to secondary browning in the subsequent concentration, sterilization and storage processes.