The response test to ultraviolet light is performed under alternating electric field. The slow response phenomenon of ZnO thin film is observed under the excitation by 254 nm ultraviolet light. And the response time constant is measured. The pure ZnO thin film and 10 at.%(atomic ratio) Al doping ZnO thin film ultraviolet detectors are prepared by using a magnetron sputtering method, in order to improve the ZnO thin film performance of UV detector. The frequency characteristic curves of the samples are tested. The results indicate that the Al doping technology can effectively restrain the slow response of the ZnO thin films, and can realize quick response of the detector to ultraviolet light with the response time constant <25 μs.