A novel process for ultra-thin (30~50μm) PIN detector fabrication has been developed. The leakage current of all our PIN detectors is found to be less than 6nA at 10V reverse bias. Compared with the leakage current before packaging, the I-V characteristics remain unchanged after the detectors are packaged. The active region get fully depleted at −3V bias, while the breakdown voltage is measured to be −664V. Finally, our ultra-thin PIN detector offers an energy resolution about 21.78KeV for 5.486MeV alpha when used in spectroscopy.