Single ZnO nanowire ultraviolet (UV) photodetector has been fabricated， and the enhancement of its UV photoresponse by polystyrene sulfate surface functionalization has been investigated. It is found that， under the same UV light irradiation， UV photoresponse of the detector with surface functionalization increases by three orders of magnitude as compared to that without surface coating. By comparing I-V characteristics of the detectors， we find that the dark conductance of the device can decrease by three orders of magnitude after surface coating while the light conductance shows little variation. The result demonstrates that one can readily enhance the UV photoresponse of the photodetector by surface functionalization.