MgxZn1-xO films (x = 0.22, 0.39, and 0.52) with a single hexagonal wurtzite structure were grown on flexible polyethylene terephthalate (PET) substrates using radio frequency (RF) magnetron sputtering technique. The piezo-phototronic properties of flexible ultraviolet (UV) photodetectors with MgxZn1-xO films were investigated by examining the relationship between their photoelectric characteristics and strain, and the impact of Mg alloying on their piezo-phototronic effect. As the tensile strain of the detectors increased from ε = 0 to ε = 0.2, the responsivity of Mg0.22Zn0.78O, Mg0.39Zn0.61O, and Mg0.52Zn0.48O flexible UV photodetectors increased by ∼ 260 %, ∼308 %, and ∼ 441 %, respectively, showing an enhanced piezo-phototronic effect with increasing Mg content. It could be attributed to the fact that the alloying process increased the piezoelectric coefficient of the MgxZn1-xO materials, thereby increasing the piezopotential. This work provides a technical approach for the preparation and performance optimization of novel flexible optoelectronic functional devices.