In order to explore whether N-acetylcysteine （NAC） can improve the stability of quercetin in cell culture medium and whether quercetin combined with NAC can show stronger antitumor activity， the real-time concentration of quercetin in culture medium was determined by high performance liquid chromatography， and the degradation equation was fitted. Then the antitumor activity of quercetin combined with NAC in vitro was evaluated by MTT method. The results showed that the degradation process of quercetin in cell culture medium accorded with the first-order kinetic characteristics （R2>0.95）. The degradation constant kobs was independent of the quercetin’s initial concentration and negatively correlated with the NAC concentration. So NAC could improve the stability of quercetin in culture medium. In addition， quercetin combined with NAC showed stronger antitumor activity of HepG-2 cells， MKN-28 cells， MDA-MB-231 cells and Caco-2 cells than the single use of quercetin or NAC. Among these four cells， NAC combined with quercetin showed strongest antitumor activity of HepG-2 cells.