Although high-strength and bubble-free Si/Si wafer pairs can be obtained by the Si/Si wafer bonding， the oxide-layer-free Si/Si bonded interface is difficult to be achieved. Thus， the Si/Si wafer bonding is difficult to be used in the field of optoelectronics. The effect of oxide layer thickness on the photoelectric characteristics （current， bandwidth， and spectrum） of wafer-bonded Si/Si p-n junction is studied. The factors which affect the performance of the Si/Si p-n junction are clarified by the simulation of carrier tunneling rate， carrier concentration， electric field， carrier velocity， and recombination rate. This may give guidance for the fabrication of ultrahigh-quality Si-based Si avalanche layer and high-performance Si-based avalanche device. The simulation results show that： with the increase of the oxide layer thickness， the carrier tunneling rate decreases， leading to the decrease of the dark current and photocurrent. The recombination rate also decreases and the carriers aggregate in the p-n junction. In addition， with the increase of the oxide layer thickness， the RC time constant of the device increases and the electric field enhances in the oxide layer， leading to the decrease of the electric field， which in turn results in the decrease of the 3 dB bandwidth.