Piceatannol is a stilbenoid, which has shown bioactivities in various cell culture models. However, its stability in cell culture medium is not clear. Here, UPLC-MS/MS was applied in situ to analyze the degradation products of piceatannol in Dulbecco's Modified Eagle's Medium (DMEM) and cell culture to investigate the compound's stability in DMEM. During the incubation with cell culture medium (at 4 and 37 °C), several piceatannol derivatives, such as an oxidation product (m/z 243.06), a reduction product (m/z 247.09), dimers (m/z 485.12 and 487.14) and trimers (m/z 727.18) were detected, which demonstrated the instability of piceatannol in cell culture conditions. To confirm if the new products during the incubation were generated due to the instability of piceatannol, ascorbic acid was added. The presence of ascorbic acid could significantly slow the degradation rate of piceatannol and the generation of piceatannol derivatives, which proved that the new products were generated by the degradation of piceatannol and indicated that the instability of piceatannol might be related to its antioxidant activity.