Self-powered heterojunction photodetectors have attracted significant attention as they can detect signals driven by a built-in electric field without external power. They are characterized by low power consumption, high sensitivity, and a fast response speed. This study employs a simple vacuum thermal sublimation method to prepare highly crystalline CuI thin films. Additionally, a smooth and dense polycrystalline film is prepared by improving the smoothness and density using confined space annealing. By building a p-CuI/u-GaN heterojunction, self-powered blue-UV photodetectors are constructed. The device responds to the blue-UV light spectrum with wavelengths <420 nm and exhibits excellent performance, including high responsivity (51 mA/W^-1), high detectivity (6.5×10¹¹ Jones), fast response times (approximately 32 ms rise time and 36 ms decay time), good stability at the 360 nm wavelength, and self-powering capabilities. This study provides a simple method for preparing high-quality semiconductor CuI films and offers new insights for the fabrication of high-performance photodetectors.