Provided is a method for using melamine and citric acid in a citric acid fermentation broth to react to form citric acid-melamine compound salt crystals used for producing sodium citrate, potassium citrate and ammonium citrate. The method includes the steps of putting the melamine into a starch fermentation broth and stirring at normal temperature to make the mixture react; filtering and washing to obtain the citric acid-melamine compound salt crystals; making sodium alkali and potash react with ammonium hydroxide respectively to form the sodium citrate, potassium citrate and ammonium citrate solutions and the solid melamine, wherein the melamine is returned to the next cycle to extract citric acid from the fermentation broth; blending the mycelium obtained by filtering with sodium phosphate calcium fodder and sweet potato powder; extruding the blending for moulding, and drying to obtain sodium phosphate calcium composite fodder; conducting concentrated crystallization on the sodium citrate, potassium citrate and ammonium citrate solutions and washing water respectively, and drying to obtain the sodium citrate, potassium citrate and ammonium citrate products.