A newly proposed method for detecting content of adriamycin in pectin–adriamycin conjugate has been developed and evaluated. The content of adriamycin was detected by selective degradation of adramycin to adriamycinone. It was realized by a two-phase reaction system (water–chloroform reaction system), in which adriamycin was quantitatively converted to adriamycinone. Therefore, the latter can be used to calculate the precise content of adramycin in the polymer drug. To develop the method, the catalyst for degradation, the extraction solvent for adriamycinone, the temperature and time of degradation, and the ratio of pectin–adriamycin conjugate were investigated. The optimal reaction condition was as follows: 30 mg of pectin–adriamycin conjugate dissolved in 25 mL of water was added to a mixture of 25 mL of hydrochloric acid (1.5 mol/L) and 50 mL of chloroform; the mixture was heated to 40 °C to react for 1.5 h; after that, the mixture was extracted with chloroform for three times, and then the organic layer was combined and, subsequently, evaporated to remove solvent. Under this condition, adriamycinone generation rate reached 99.87%. The quantitative method was evaluated for linearity, the limit of detection (LOD) and limit of quantitation (LOQ), recovery, accuracy, robustness, and precision. The recoveries were between 99.47% and 101.07% with relative standard deviation <1.23%. The LOD and LOQ were 0.06 and 0.17 μg/mL, respectively. Compared to the traditional ultraviolet (UV) detection, this method is considered to be more precise for detecting content of adriamycin in its polymer conjugate.