Ultrathin PIN Detectors have been applied in radiation detection for particle identification and etc. In this paper, we present simulation research on the structure of ultrathin Si PIN detector based on bonding technology by using Sentaurus TCAD tool. The normal structure and reverse structure of ultrathin Si PIN detector are simulated and compared. The reverse current of detector and electrical field distribution are analyzed. It is found that the reverse current of the reverse structure increases fast when the voltage exceeds a threshold value. It is explained by considering the parasitic MOS structure. This effect can be reduced by increasing the thickness of buried SiO2.