Thin MgO nanostructure layer was grown on a-plane GaN film in this study using low temperature aqueous method. The thin MgO layer showed uniform sheet-like morphology. Meanwhile, ultraviolet (UV) photoconductive detector based on MgO/a-GaN heterostructure was fabricated by simple way. The obtained detector displayed excellent UV sensing properties. Results showed that the thin MgO nanostructure layer can effectively decrease the dark current and passivate the surface defects. The facile method will provide a new route to adopt nanostructures for applications in enhancing the performance of GaN-based UV detector.