The invention relates to a preparation method of antibiotics, and aims to provide a preparation method of an instantly dissolved azithromycin oral suspension. The method is simple in preparation process, the obtained instantly dissolved azithromycin oral suspension also has the characteristic of stable quality of a dry suspension, the shortcomings that dosage cannot be further subdivided accurately and the taste is poor are overcome, and the clinical application prospect is good. According to the technical scheme, the preparation method of the instantly dissolved azithromycin oral suspension comprises the following steps: (1) preparing powder for suspending of azithromycin, weighing 600 mg of azithromycin oral crude drugs, 1,750 mg of a diluent and 150 mg of sodium bicarbonate, and then uniformly mixing and packing; (2) preparation of a blank solution for suspending: (a) weighing blank solution raw materials for suspending; and (b) preparing the blank solution by a corresponding preparation process.