A ZnO ultraviolet photodetector with a metal-semiconductor-metal (MSM) structure was successfully prepared by the radio frequency magnetron sputtering technology. The bias voltage dependence of the responsivity and cutoff wavelength of the detector is studied. With the increase of bias voltage, the responsivity of the detector gradually increases and tends to saturate and the response cutoff wavelength of the detector is redshifted by 12 nm. This is attributed to the broadening of the depletion layer and the tilt of the bandgap caused by the field. In this work, an effective method is proposed to control the cutoff wavelength of the detector by an external bias voltage, which is of great significance to the further investigation and application of ultraviolet photodetectors.