Remifentanil is a new, ideal, narcotic analgesic commonly used in surgery, which has the advantages of rapid effect, short half-life, easy to control, quick recovery after the operation etc. After entering the human body, remifentanil is mainly metabolized by the kidneys. Then remifentanil acid is formed after de-esterification metabolism. Therefore, it is necessary to study the structure of remifentanil and its metabolite remifentanic acid. At present, only the structure of the prototype drug has been studied, but the structure of its metabolite remifentanic acid has not been reported. Methods For the first time, this study established ultraviolet spectra, infrared spectra, nuclear magnetic resonance (NMR) spectra (i. e., 1H NMR, 13C NMR, 1H-1H COSY, 1H-13C HSQC, 1H-13C HMBC and DEPT) and mass spectra for the chemical structure identification of remifentanil acid. The ultraviolet spectrum showed the aromatic structure and the information of the conjugated system of remifentanil acid. The maximum absorption in the vicinity of the ultraviolet end and 254.0 nm respectively corresponded to the E2 and B bands of the substance, which showed the presence of the benzene ring characteristic structure in the molecule. The infrared spectrum test was performed by preparing the remifentanil acid samples by the potassium bromide tablet method. The infrared spectrum was used to analyze the peaks of the functional group vibrations of remifentanil acid. All the 1H NMR and 13C NMR chemical shift signals of the compound were assigned rationally by NMR spectroscopy (including 1H NMR, 13C NMR, 1H-1H COSY, 1H-13C HSQC, 1H-13C HMBC and DEPT). Mass spectra were performed by positive electrospray ionization, and the mass-to-charge ratio (m/z) was 362.98, 331.02, 303.10, 259.09 and so on, which was consistent with the molecular weight of remifentanil acid and corresponding to the structural characteristics of remifentanil acid. The results showed that the above analytical data of a variety of spectral techniques coincided with the structure of remifentanil acid. Spectral analytical data is shown to be consistent with the structure of remifentanil acid, which can be used in the study of the structure of remifentanil acid and provide a reference for the study of its quality and purity.