The present invention relates to a liquid fertilizer and its preparation method. It is made up by using concentrated sugar liquid, urea, potassium chloride, potassium dihydrogen phosphate, zinc sulfate, cupric sulfate, manganous sulfate, ferrous sulfate, calcium sulfate, boric acid and magnesium sulfate through the processes of solid-liquid separation, fermentation treatment, concentration, refining, mixing materials, filling, inspection and packaging. It is a high-efficiency composite fertilizer, not only possesses organic nutrient components, but also has effective elements and trace elements, can promote plant growth and development, raise plant resistance to diseases and drought, can improve soil structure and raise utilization rate of fertilizer, etc..