The invention discloses low-calorie fermented fruit and vegetable juice and a preparation method thereof. The juice consists of the following components in parts by weight: pullulan, stamen and pistilsugar, D-mannose, fructo-oligosaccharide, L-tagatose, L-fucose monosaccharide, beta-benzpyrole propanoic acid benzyl ester, 3-sieboldin, oxyresveratrol, beta-methyl escultin, dihydrocoriandrin, fruits and vegetables, aleppo avens, twig of winged euonymus, yerbadetajo herb, stabilizers, emulsifiers and antioxidants. The fruits and vegetables include passion fruits, avocado, naseberries, lychee, hawthorn fruits, spinach, celery, carrots, taro and lettuce. The fruits and vegetables are subjected to pulping, enzymolysis, enzyme inactivation and sterilization so as to obtain fruit and vegetable juice; then low-calorie sugar is added for fermentation treatment so as to obtain fermentation liquor; and then stirring, concentration and homogenization treatment are carried out to obtain the fermented fruit and vegetable juice with low calorie. The juice has good nutrients, a pure taste and strong stability, and can balance blood sugar.