The invention provides a preparation method of dezocine injection. The preparation method comprises the following steps: selecting water for injection as the solvent, adding with lactic acid, carrying out stirring for dissolving, and carrying out heating till the temperature is 35-50 DEG C; adding with the medicine, namely, dezocine, and carrying out stirring for clarification; adjusting the pH by adopting sodium hydroxide; and enabling the obtained solution to pass through carbon, carrying out filling, and carrying out high-temperature sterilization, thus obtaining the dezocine injection. For the dezocine injection prepared by adopting the method provided by the invention, the related substance of the medicine is not increased during the preparation process of the injection, meanwhile, three auxiliary materials, namely, sodium pyrosulfite, propylene glycol and sodium chloride are correspondingly reduced, and thus the stability of the injection is further enhanced; moreover, the variety of the used auxiliary materials is reduced, so that the risks generated due to the side effects of clinical medication are reduced, and the preparation method is suitable for the large-scale industrial production.