A series of metal element (Al, Fe, Mg and Ni)-doped ZnO (M-ZnO) photocatalysts have been successfully synthesized on graphene-coated polyethylene terephthalate (GPET) flexible substrate via the hydrothermal method. The effects of doped metals in ZnO were also studied on the crystal structure, morphology and photocatalytic performance. The photocatalytic experiment results indicated that, compared with Al-, Mg- and Fe-ZnO/GPET photocatalysts under ultraviolet (UV) light irradiation, Ni-ZnO/GPET had better photocatalytic activity, and the degradation rate of methylene blue (MB) was 81.17%. Meanwhile, the mechanism of enhancing the photocatalytic activity of metal element-doped ZnO is also discussed. It is concluded that, after doping with metal elements, electrons and holes are prevented from recombination by trapping electrons of the ZnO/GPET conductive band, thereby improving the photocatalytic activity.