The invention provides a watermelon enzyme liquid fertilizer and a preparation method thereof, belonging to the technical field of bio-organic fertilizer preparation and utilization. The watermelon enzyme liquid fertilizer is prepared from the following components in percentage by mass: 45-68% of watermelon, 1-5% of lemon, 2.5-8% of grapefruit, 3.5-15% of tomato, 1-5% of pineapple, 0.3-1% of yeast, 0.5-3% of urea, 0.7-2% of potassium sulfate, 4-6.5% of calcium superphosphate, 0.15-0.5% of zinc sulfate, 0.1-0.5% of borax, 0.3-0.8% of brown sugar or molasses, 5.5-20% of fresh plant and 3-20% of water, wherein the total percentage of all components is 100%. The product is rich in multiple enzymes, vitamins, minerals, amino acids and trace elements; by adopting the product, the fruits and vegetables grow vigorously, the plant root system is well developed, the pest and disease damage is effectively reduced and prevented, the microbial population of soil is enlarged, the soil is improved, the yield and quality of fruits and vegetables are increased, and the product is economical and environment-friendly.