Salinomycin sodium (SAL-Na) is a type of antibiotic chemotherapeutic drugs with the potential to treat cancer stem cells. The assay method of SAL-Na included in the pharmacopoeia is a microbiological method, which is not suitable for the rapid detection in daily scientific research. Besides, the assay methods of SAL-Na reported by literature are not suitable for quantification due to the interference of various excipients. Consequently, the deep study on biological mechanism of SAL-Na is hindered by its assay method. In the present study, we aimed to establish an ultraviolet visible (UV-vis) spectrophotometric method to determine the content of SAL-Na in the liposomes. The first approach was a UV spectrophotometry, in which SAL-Na was dissolved in ethanol and then detected at 287 nm. Although the standard curve measured at 287 nm by UV method had good linearity, the quantification limitation was too high to meet the requirement in determining SAL-Na in the liposomes. In addition, the membrane materials in the liposomes severely affected the measurement. The second one was an improved UV-vis spectrophotometry by vanillin derivatization. In this method, SAL-Na was dissolved in 95% ethanol, mixed with vanillin test solution and heated at 72 °C for 40 min for derivatization. After cooling down to room temperature, the solution was detected using UV-vis spectrophotometer at 526 nm. This method could be used to accurately determine the content of SAL-Na at lower concentration, and the absorbance value was stable for 5 d at least. Moreover, the membrane materials of the liposomes did not affect the absorbance of SAL-Na at 526 nm. The precision and recovery studies demonstrated that the vanillin derivatization approach was stable and precise in assaying SAL-Na. In conclusion, the UV-vis spectrophotometry by vanillin derivatization could be used for measuring SAL-Na in the liposomes, thereby laying a foundation for deep study of the biological mechanism of SAL-Na in the liposomes.